key: cord-0881972-d6a35prz authors: Cascini, Fidelia; Pantovic, Ana; Al-Ajlouni, Yazan A.; Failla, Giovanna; Puleo, Valeria; Melnyk, Andriy; Lontano, Alberto; Ricciardi, Walter title: Social media and attitudes towards a COVID-19 vaccination: A systematic review of the literature date: 2022-05-20 journal: eClinicalMedicine DOI: 10.1016/j.eclinm.2022.101454 sha: dfb51af41e1dc6c0fa9a443bbcc2d99ac19ee850 doc_id: 881972 cord_uid: d6a35prz BACKGROUND: Vaccine hesitancy continues to limit global efforts in combatting the COVID-19 pandemic. Emerging research demonstrates the role of social media in disseminating information and potentially influencing people's attitudes towards public health campaigns. This systematic review sought to synthesize the current evidence regarding the potential role of social media in shaping COVID-19 vaccination attitudes, and to explore its potential for shaping public health interventions to address the issue of vaccine hesitancy. METHODS: We performed a systematic review of the studies published from inception to 13 of March2022 by searching PubMed, Web of Science, Embase, PsychNET, Scopus, CINAHL, and MEDLINE. Studies that reported outcomes related to coronavirus disease 2019 (COVID-19) vaccine (attitudes, opinion, etc.) gathered from the social media platforms, and those analyzing the relationship between social media use and COVID-19 hesitancy/acceptance were included. Studies that reported no outcome of interest or analyzed data from sources other than social media (websites, newspapers, etc.) will be excluded. The Newcastle Ottawa Scale (NOS) was used to assess the quality of all cross-sectional studies included in this review. This study is registered with PROSPERO (CRD42021283219). FINDINGS: Of the 2539 records identified, a total of 156 articles fully met the inclusion criteria. Overall, the quality of the cross-sectional studies was moderate – 2 studies received 10 stars, 5 studies received 9 stars, 9 studies were evaluated with 8, 12 studies with 7,16 studies with 6, 11 studies with 5, and 6 studies with 4 stars. The included studies were categorized into four categories. Cross-sectional studies reporting the association between reliance on social media and vaccine intentions mainly observed a negative relationship. Studies that performed thematic analyses of extracted social media data, mainly observed a domination of vaccine hesitant topics. Studies that explored the degree of polarization of specific social media contents related to COVID-19 vaccines observed a similar degree of content for both positive and negative tone posted on different social media platforms. Finally, studies that explored the fluctuations of vaccination attitudes/opinions gathered from social media identified specific events as significant cofactors that affect and shape vaccination intentions of individuals. INTERPRETATION: This thorough examination of the various roles social media can play in disseminating information to the public, as well as how individuals behave on social media in the context of public health events, articulates the potential of social media as a platform of public health intervention to address vaccine hesitancy. FUNDING: None. In 2019, The World Health Organization (WHO) listed vaccine hesitancy as one of the top 10 threats to world health. 1 In the context of public health responses, vaccination campaigns are considered to be one of the most successful public health interventions and a cornerstone for the prevention of communicable infectious diseases. While vaccine hesitancy is perceived to be a long-standing problem, it has recently assumed great urgency in light of the coronavirus disease 2019 (COVID- 19) pandemic. 2 Vaccine hesitancy, defined as delay in acceptance or refusal of vaccination despite availability of vaccination services, may be the product of an array of factors and held views. 3 Emerging research in the literature has investigated such factors in relation to COVID-19 vaccine hesitancy and demonstrated that hesitancy may stem from concerns relating to vaccine efficacy, safety, side effects, convenience, price, beliefs that the vaccine is not necessary, the idea that testing for the vaccine was insufficient and that the pace of its development was too quick, as well as the financial motivation of the authorities/pharmaceutical companies. 4 Among the many barriers that contribute to vaccine hesitancy and were identified globally, a recurrent theme continues to be that hesitancy is due to misinformation regarding the benefits and the adverse effects of vaccines. Given that people increasingly refer to the internet and social media networks to seek information, emerging research in the literature has recently placed special emphasis on investigating the association between social media usage and attitudes towards public health interventions, specifically COVID-19 vaccinations. 5−8 The ongoing mitigating efforts to control the spread of SARS-CoV-2, including physical distancing, quarantine requirements, and travel bans across the world, may intensify the use of social media as individuals try to remain connected while physically apart. This increased reliance on social media networks may be concerning due to its ability to spread misinformation and contribute to increased hesitancy, especially among populations who may be vulnerable to increased hesitancy rates (e.g., those in low socioeconomic status groups). 9, 10 Previous research has shown that vaccine hesitant groups on social media have an alarming footprint and report that anti-vaccination messages are large proportions of the content about vaccines on popular social media sites, 11−18 which engenders more Evidence before this study Prior to conducting our systematic review, we searched PubMed and Web of Science without language restrictions and using keywords such as "social media", "COVID-19 00 and "vaccine" to identify previous evidence in the form of a systematic review on our topic of interest. Prior to and during the conduction of this systematic review, we identified several other systematic reviews that have addressed the issue of social media and attitudes towards a COVID-19 vaccination. Prior research has demonstrated the role that social media plays as a platform for mass media and information transmission. Additionally, given that many patients and individuals refer to social media as their source of information, emerging research has been placing considerable emphasis on how social media can influence attitudes toward public health interventions and campaigns. It has been shown that vaccine hesitancy plays a major role in affecting vaccination rates, as well as the potential in reaching herd immunity in the context of the COVID-19 pandemic. Consequently, an abundance of research has emerged in the literature studying the potential role of social media in shaping attitudes regarding COVID-19 vaccination, as well as social media's role as an intervention platform to address vaccine hesitancy. However, to date and to the best of our knowledge, no systematic review has been published synthesizing all the evidence relating social media and vaccine attitudes. Upon those bases, and on March 13th, 2022, we conducted a systematic review of the literature using developed keywords. Studies that reported outcomes related to coronavirus disease 2019 (COVID-19) vaccine (attitudes, opinion, etc.) gathered from the social media platforms, and those analyzing the relationship between social media use and COVID-19 hesitancy/acceptance were included. Studies that reported no outcome of interest or analyzed data from sources other than social media (websites, newspapers, etc.) will be excluded. The Newcastle Ottawa Scale (NOS) was used to assess the quality of all cross-sectional studies included in this review. Our systematic review, carried out according to best practices, included 156 peer-reviewed articles, most of which have been published in the past year. Overall, our review exemplified the association between social media reliance for information and hesitant attitudes, as well as a demonstrated strong potential for social media as an intervention area to address vaccine hesitancy and aid in reaching herd immunity globally. The evidence we found have important implications for public health research and practice. Firstly, our findings exemplify that understanding social media usage patterns can provide an opportunity for targeted intervention. Additionally, the findings of this review demonstrate that there is a strong potential for polarized views to be amplified using social media, which presents an opportunity for targeting misinformation. Furthermore, utilizing social media to understand public's sentiment allows for tailored and targeted intervention, specific to the current views held by the public. Finally, given that specific events trigger people's emotions regarding vaccination, it is crucial for the government to adjust vaccination policies promptly in response to the public health events to promote massive vaccination via dynamic monitoring public sentiments. user engagement. 19, 20 Another important point is the existence of echo chamber effect on social media platforms, by gathering individuals and surrounding them by likeminded people in terms of ideological orientation. 21 For example, such echo chambers existon Facebook, and it is confirmed that pro-and anti-vaccination attitudes polarize the users' opinion. 22 Therefore, studying networked communities can help in understanding online discussions/ public opinions on vaccine hesitancy and exploring how they can impact society and science communities. 23 On the other hand, it is recognized that utilizing social media data can offer advantages over traditional survey methods in this critical times, by enabling timely monitoring of public attitudes/opinions, which altogether supports the notion of analyzing social media data with the aim to better understand vaccination intentions and attitudes regarding the ongoing immunization campaign. 24 Although the latest updates derived from surveys/ polls conducted around the world indicate that vaccine hesitancy shows a decreasing trend over time, 25 this issue deserves further attention and deeper investigation in order to enhance the willingness to vaccinate even more, and to prevent such phenomenon in case of similar future scenarios. Upon these bases, this systematic review of the literature sought to summarize the available data in the literature on the various associations between social media usage and attitudes toward COVID-19 vaccination campaigns worldwide. To the best of our knowledge, this is one of the first and most extensive systematic review to be conducted on this extremely important association. This systematic review of the literature was performed according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Several databases were selected for searching potential eligible articles: PubMed, Embase, ISI (Web of Science), Scopus, PsycNET, Medline and CINAHL. We also performed a manual search of reference lists of included articles. The initial systematic search was performed in September 2021, and as part of the review process we performed the updated systematic search of all databases in March 2022. The protocol related to this systematic review is registered with PROSPERO (CRD42021283219). The search criteria were defined by combining the keywords related to the general exposure of interest (social media), the outcome of interest (vaccination) and was further focused by adding keywords related to the specific disease . The detailed search strategy is presented in Supplementary Table S1 . We included studies that reported any outcome related to COVID-19 immunization campaigns gathered from social media (related to opinions, attitudes, etc.). We also included studies that determined the rate of vaccine hesitancy/acceptance in a population, and reported the associations observed with social media use. Studies that analyzed data from sources other than social media (websites, newspapers, etc.) were excluded. Only studies published in the English language published before September 2021 were included. The detailed definition of the inclusion/exclusion criteria is given in Supplementary Table S2 . After performing the systematic search of all electronic databases, we retrieved the articles, removed the duplicates, and imported the references in Rayyan. 26 As illustrated in Figure 1 , titles and abstracts were screened by two reviewers independently (AL and VP). All conflicts were resolved by contacting the third reviewer (AP). After this step, we performed the full-text assessment in the same manner, thereby identifying the final list of eligible articles. The data extraction was performed by two reviewers (A.P. and Y.A.A), and every disagreement was resolved by discussion with the third reviewer (F.C.).The data from the eligible studies were extracted in a pre-defined Excel sheet, defined by the following columns: (1) the name of the first author, (2) the geographical context, (3) the aim, (4) the study design (5) the study period when the assessment/analysis was performed, (6) the study population or study content, (7) vaccination status (where applicable), (8) the main results related to the reported associations/correlations between social media use and vaccine acceptance/hesitancy, (9) the main results related to the content gathered from the social media that were related to COVID-19 vaccination. After summarizing all the data in the Excel sheet, we were able to identify all studies that fulfilled the inclusion criteria, and to remove those that initially passed the full text screening but were deemed inadequate to be included in the systematic review. The data are presented in a tabulated form and were either presenting associations between social media utilization and vaccine hesitancy, reporting analyses of public opinion on COVID-19 vaccines, analyzing the sentiments of the population regarding the current immunization campaigns and lastly, or reporting the change and fluctuations in the vaccine-related discussions caused by some driver/trigger events. Due to the nature of the data, we were not able to perform any visual presentations. We evaluated the quality of only cross-sectional studies that reported the associations between vaccine intentions and social media use. The quality of these studies were evaluated with the Newcastle-Ottawa Scale (NOS) for non-randomized studies. 27 NOS uses a star system, where a study can have a maximum of 9 stars after evaluating for 3 main categories: selection (for which a study can have a maximum of 4 stars), comparability (a maximum of 2 stars) and outcome (a maximum of 3 stars). The initial systematic search of electronic databases identified a total of 4408 studies. Overall, a total of 155 studies were included as part of this updated systematic search, and the full process for both searches is presented on a separate PRISMA flow-chart ( Figure 1) The included studies were categorized into the following four categories: (1) cross-sectional studies reporting the association between reliance on social media and vaccine hesitancy and/or acceptance. (2) studies that performed thematic analyses of extracted social media data, thereby reporting Articles discussions related to COVID-19 vaccine intentions/opinions; (3) studies that explored the degree of polarization of specific social media contents related to COVID-19 vaccines (the degree of positive vs. negative sentiments); (4) studies that explored the fluctuations of vaccination attitudes/opinions gathered from social media depending on specific events that were identified as trigger events. The studies from the first category were all cross-sectional in nature (Table 1) , and mainly employed anonymous online surveys for assessments. From a total of 61 studies (62 records), USA had the highest numbers of studies per country (n = 11). Two studies were conducted across multiple countries, while no studies were reported from South America or Africa. The other 3 categories of studies mainly focused on some specific (or several specific) social networks. Notably, Twitter was the predominant site utilized in these studies -62 out of 94 studies that analyzed COVID-19 vaccination social media content used solely Twitter as the main source of data. These studies also used tools for data extraction (such as R libraries to assess the Twitter premium application programming interface service and collect the posts). Some of the studies focused on posts obtained from a specific location, thereby exploring the state of mind of one population, while others did not focus their analysis on any geographical basis. Almost all studies scraped posts published by the users, some studies focused on organizations, 28, 29 while there were several studies that analyzed users' comments posted on news media pages. Out of total of 155 studies, quality was evaluated only in cross-sectional (n = 61, Supplementary Table S3 ). It should be noted that the study published by Luo et al., 2021 and are separate records of the same study, thus we reported only the quality of the study reported by Luo and colleagues. Overall, the quality of the cross-sectional studies was moderate − 2 studies received 10 stars, 5 studies received 9 stars, 9 studies were evaluated with 8, 12 studies with 7, 16 studies with 6, 11 studies with 5, and 6 studies with 4 stars. The associations between social media use and vaccine hesitancy/acceptance The main results and characteristics of studies assessing the relationship between vaccine intentions-attitudes and utilization of social media are presented in Table 1 . The studies reported variable conclusions, however, the number of studies that observed a negative association between social media use and vaccine acceptance was higher. One group of studies found a direct positive association between the utilization of social media and vaccine acceptance, with 2 of the reports referring to the same study. 30−38 These positive results were observed among a young population divided into medical and non-medical personnel, with the majority of them being women (mean age of 28 years), among nursing students aged 18−23 years, participants aged 12−18 years, among older adults (older than 65 years), adult patients with neurological disorders, adults who have been diagnosed with HIV or AIDS, young nurses. Interestingly, parents who were healthcare workers and were more reliant on social media were also more inclined towards vaccinating their children against COVID-19. 39 There were also studies that reported a positive indirect association between social media utilization and vaccine acceptance. For example, Zhang et al., 40 observed that the association between social media usage and attitudes toward vaccination depends on the type of content the individuals are exposed to (positive content was positively associated with the intention to vaccinate and vice versa), as observed in their sample of factory workers. Another study highlighted the importance of doctors as social media influencers, which was also positively associated with higher vaccine acceptance, as observed in a study conducted on women of reproductive age. 41 There was the group of studies that did not observe any effect of social media exposure on the intention to vaccinate against COVID-19 (n = 8). The rest and the great majority of the studies (n = 41) reported that social media imposed a negative impact on the participants' intention to vaccinate. These negative associations were confirmed among all age groups of people, originating from different geographical locations from all continents (except Australia), and who were characterized as either students (medical and non-medical), healthcare workers, the general population, women of reproductive age, pregnant women, Medicare beneficiaries. Finally, Jennings and co-workers distinguished the different types of social media and their relations with vaccine acceptance. Compared with non-users, YouTube users showed a lower willingness, while those relying on Facebook and Twitter showed a higher intention to receive the COVID-19 vaccine. 42 Themes emerging from discussions among social media users The summary characteristics and results from studies that analyzed posts extracted from social media are presented in Frequency of exposure to positive information related to COVID-19 vaccinations on social media was positively correlated with positive attitudes (r = 0.083; p<0.001), perceived subjective norm (r = 0.101; p<0.001), and perceived behavioral control (r = 0.064; p = 0.004) related to a COVID-19 vaccination. A negative correlation was found between social media exposure and negative attitudes toward a COVID-19 vaccination (r=−0.090; p<0.001). Table 1 : Summary characteristics and main results of studies that reported the association between the reliance on social media and COVID-19 vaccination intentions. Abbreviations: NR − not reported; OR − odds ratio; CI − confidence interval; SD − standard deviation; AOR − adjusted odds ratio; RR − risk ratio. The tweets revealed vaccine support through vaccine affirmation, advocacy through reproach, a need for a vaccine, COVID-19 and racism, vaccine development and efficacy, racist vaccine humor, and news updates. Vaccine opposition was demonstrated through direct opposition, vaccine hesitancy, and adverse reactions. Conspiracy and misinformation included scientific misinformation, political misinformation, beliefs about immunity and protective behaviors, and race extermination conspiracy. Equity and access focused on overcoming history of medical racism, pointing out health disparities, and facilitators to vaccine access. Representation touted pride in development and role models, and politics discussed the role of politics in vaccines and international politics. YouTube videos posted on Twitter Following the World Health Organization's declaration of the COVID-19 outbreak as a public health emergency of international concern, anti-vaccination frames rapidly transitioned from claiming that vaccines cause autism to pandemic conspiracy theories, often featuring Bill Gates. Content analysis of the 20 most tweeted videos revealed that the majority (n = 15) opposed vaccination and included conspiracy theories. Their spread on Twitter was consistent with spamming and coordinated efforts. These YouTube, Twitter, Facebook and Instagram This study identified twenty-two narrative tropes coded, out of which, four key common narrative tropes were: (1) "Vaccine Injury'' -these injuries were often vaguely described and only infrequently accompanied by claims to an actual diagnosis of malady. (2) "Corrupt Elite" -this offered a standard populist appeal in which an innocent put disempowered ); concern about side effects/long term health implications. Physical opportunities category -lack of will to pay for the vaccine, refuse mandatory vaccination, concern over availability of information. Behavioral intentions category -consideration that the disease was not severe or lifethreatening and the vaccine was not effective (because of low efficiency and mutation of the virus), consideration that the rushed vaccine to be more harmful than COVID-19. The campaign aims to defy all western vaccines and stress the importance of Russia's contribution to COVID-19 overcoming. Specifically for Ukraine, they also defamed the hypothetical Ukrainian vaccine promised by Zelenskiy. 5)Ukrainian social media users campaign -this campaign was conducted by bloggers and journalists and made fun of the Russian vaccine and emphasized that it was not tested enough, so it is potentially dangerous Twitter Topics related to negative attitudes: safety issues of the COVID-19 vaccines, unknown side effects, rushing the development process. Some users even questioned the existence of COVID-19 or COVID-19 vaccines and indicated a lack of trust in the government or scientists. Some users feared that the virus mutation would render the vaccine ineffective and thus had negative attitudes toward vaccines. Topics related to positive attitudes: relevant key terms included "safe," "stay," "end," pandemic," "news," "effective," "trial," "continue," and "hope." This indicates that some positive attitudes might be derived from news of effective trial results and some users hoped that COVID-19 vaccines could end the pandemic. Relevant terms for topic 4 were "hope," "normal," "life," "return," "start," "new," "world," and "great." Some users expressed positive attitudes toward vaccines because of the desire to return to a normal life. Twitter Anti Vaccine Narratives -3 distinct communities identified; all of them contained anti vaccine keywords, but with different focuses on topics. The largest topic community focuses on debunked claims around the conspiracy narrative that the vaccine is a plot by rich people to reduce the world population. The second topic community mostly focuses on vaccine safety, as hashtags such as #doctorsspeakup, #vaccinesafety, and #vaccineinjury appear often. The smallest topic community contains a mixture of various hashtags that range from strongly antivaccine, such as #informedconsent, #learntherisk, and #vaxxed, to some neutral hashtags, such as #vaccine, to some provaccine hashtags, such as #vaccineswork. Paul and Gokhale 2020 74 Public posts The anti vaccination community is vociferous in opposing the vaccine, spreading misinformation, spinning conspiracies and whipping hysteria. Significant hesitation about the safety of the Covid-19 vaccine is also expressed in particular because of its rapid deployment. The pro-vaccination community counters this opposition by pointing to prior successes of immunizations as well as by mocking the anti-vaxx attitudes. A comparison of the social features of the anti-vaccination and pro-vaccination tweets suggests that the anti-vaxx community has gained steam on social media platforms and is better connected than the pro-vaccination community, which may lead to a penetration of discordant information through the online world. Five themes are new and specific to COVID-19 anti-vaccine sentiment. They argue that these vaccines have been developed too quickly without a proper test. They also suggest that vaccines do not exist despite the information from pharmaceutical companies and governments. Some users remind about the development of the swine flu vaccine. This vaccine was also developed quickly, but, over time, it caused more side effects than expected. On the other hand, conspiracy theories suggest that vaccines against COVID-19 existed before the official announcement, even before the pandemic. The last specific argument implies that people should avoid vaccination because no one takes responsibility for the potential side effects of the vaccine. Only 15% of the comments were positive, while 85% were negative. The most popular anti-vaccine arguments in our dataset are: the lack of trust in the government, the danger of vaccines, and the lack of faith in the existence of an effective vaccine. A share of pro-vaccine comments have increased from 7% to 22% during the event of first vaccination. In total 8100 posts in NAVER and 5291 posts in Instagram were sampled through web crawling. The findings revealed a negative perception of COVID-19 vaccines; of the words crawled, the proportion of negative words for AstraZeneca was 71.0% (476/670) and for Pfizer was 56.3% (498/885). Among words crawled with "vaccine," "good" ranked first, with a frequency of 13.43% (312/2323). Meanwhile, "side effect" ranked highest, with a frequency of 29.2% (163/559) for "AstraZeneca," but 0.6% (4/673) for "Pfizer." With "vaccine," positive words were more frequently used, whereas with "AstraZeneca" and "Pfizer" negative words were prevalent. Twitter This study found that such "belief echoes" do exist among Twitter users in relation to the perceived safety and efficacy of the COVID-19 vaccine as well as the vaccination hesitancy for themselves and their children. These "belief echoes" manifested as skepticism of adequate COVID-19 immunization particularly among Republicans and Independents as well as female Twitter users. Surprisingly, we found that the belief echoes are strong enough to preclude adult Twitter users to receive the COVID-19 vaccine regardless of their education level. Public posts Twitter There was no independent health expert in the USA tweet terms examined. The study identified the presence of non-scientific influencers from the business community in Ireland and South Africa which could be a potential cause for concern on this sensitive topic, although mainstream news sources may also be non-expert influential commentators. 52, 54, 55 distrust in the efficacy of the vaccines within the context of rapidly emerging mutant viruses, 46,52 distrust in the production and transport of the vaccines, 56, 57 and controversy around the vaccines' ingredients (toxins, mercury). 43 Topics categorized as misinformation 58 and conspiracies were also frequent and included population control via microchips that was planned by Bill Gates, 46, 52, 54 adverse effects of vaccination such as cancer and sterility, and many more. 52 A few studies reported themes such as distrust in government and their activities in handling the pandemic, as well as distrust in major health regulatory bodies (e.g., Food and Drug Administrationor Centers for Disease Control and Prevention). 45, 49, 57 Topics that were discussed as providing justification against vaccination and were related to COVID-19 specifically included considerations that the severity of the disease was not so high, 59 comparisonswith other diseases (such as Ebola or H1N1), as well as the opinion that one does not belong to the category that is athigher risk of more severe symptoms. 46, 54 Apart from vaccine-hesitant topics, a few studies identified those that support the current immunization campaign, and the reported reasons are mainly trust in science and healthcare professionals, trust in vaccine as a solution, 29 the reliance on peer-reviewed publications for making informed decisions. 51 Finally, there were several studies that were not geo-located that explored vaccine-related topics in general that reported noteworthy observations. Namely, the peaks of the topics were affected by the events reported by the media and posted on social media. 60 Further, vaccine-supportive tweets showed temporal variations over time, while those related to barriers remained mainly constant through time. 61 After conducting the updated search,we were able to locate new studies that focused social data mining on specific regions, thereby providing more insights into themes that were specific to a certain location. However, the topics identified as vaccine-hesitant were unique regardless of the location − such as mistrust into the manufacturing process, science, and reliance on conspiracy theories in Turkey, 62 distrust in the scientific and manufacturing process in China, 63 non-necessity of the vaccine, concerns over safety and side effects, in Canada. 29 These topics were mainly identified through our initial search conducted up to September 2021, and we could observe that themes that were discussed by the public and were oriented against vaccination were repetitive throughout different geographical locations throughout different study periods. Furthermore, eighteenstudies reported different topics providing a difference between the anti-and pro-vaccination discussions. One group of studies focused on describing the discussions and reasons provided by the opposing parties -vaccine refusers mainly focused on side effects and safety coupled with conspiracy theories and misinformation. 64−69 Alternatively, vaccine supporters believed that the immunization would help return life to normal and it gave them hope in ending the pandemic, as well as provided a sense of safety. 53,70−73 The second group of studies focused on exploring the strategies employed by these two opposing communities. The anti-vaccination community was described as spreading misinformation, spinning conspiracies andinstigatinghysteria. 74 It was also described as spreading a nonfocused discussion thereby appealing to a wider range of groups (for example, by providing suggestions for alternative remedies for those seeking them). 75 Johnson and co-workers observed that anti-vaccination clusters are better connected with the undecided clusters in the main online network, while pro-vaccination clusters remain peripheral. Thus, theanti-vaccination clusters predict their domination in a decade, as observed from the analysis of posts obtained from Facebook pages. 76 On the other hand, pro-vaccination communities reportedly act by trying to convince skeptics by touting prior successes of immunizations and expressing concern because of the rise of "anti-vax" movement on Facebook and Twitter, however they also mock and make insults of anti-vax communities in disrespectful and derogatory tones. 74, 77 Again, anti-vaccination communities were regarded as being better connected than pro-vaccination clusters, as suggested by the comparison of the social features of their tweets. 74 When one study analyzed the 2000 most active Twitter accounts in the vaccine discourse from 2019, it reported that up to 45% were identified as opposing vaccination, while only 24% were in favor. 71 Finally, one study provided an insight about what type of actor is behind the anti-vaccination movement. 78 identified that it was primarily led by political and non-medical Twitter users, with less than 10% of these users being from the medical community. This also points to that problem of health care professionals' inactivity in addressing COVID-19 misinformation and spreading scientific evidence in order to combat this issue. Specifically in Turkey, an analysis of pre and post COVID-19 tweets showed that the number of anti-vaccine supporters was 22 greater than of those who are pro-vaccine, and the increase in the number of anti-vaccination supporters after COVID-19 has begun was huge (from 1.7% to 26.51%). 79 One study pointed to the fact that Donald Trumpwas the main driver of vaccine misinformation on Twitter, before his profile was suspended. 80 On the other hand, an analysis of tweets containing #yomevacuno hashtag found that healthcare professionals comprise only 11.38% of users, generating only 6.35% of impressions, and that although the traffic information produced by them is not significant compared with institutions it is compared with regular users. 81 Ninestudies were classified as miscellaneous regarding the thematic discussions obtained from social media data. Lee and co-workers observed a high negative sentiment towards COVID-19 vaccines from posts obtained from Korean Instagram and NAVER users -namely the proportion of negative words for AstraZeneca vaccine was 71%, while for Pfizer it was 56.3%. 82 Another study that analyzed public Twitter posts observed that the positive sentiment towards vaccines in general was 20 −25%, negative was around 10% and the rest was neutral. 83 Interestingly, the USA tweets revealed no existence of an independent health expert, while nonscientific influencers from the business community were observed to be active in the Twitter community in Ireland and South Africa. 84 Only one study conducted an online survey on 319 participants to explore the phenomena of belief echoes and it found that they can be strong enough to affect adult Twitter users' intention to vaccinate regardless of their education level. 85 Gawel and co-workers explored did Pope Francis words have an effect on Catholics' decisions to accept or refuse the COVID-19 vaccination, and although it is estimated that it did reach the public its' effectiveness is quite questionable. 86 The degree of polarization of views / content regarding COVID-19 vaccination on social networks There were 28 studies that examined the degree of polarization of social media content related to COVID-19 vaccines ( Table 3 ). The degree of polarization mainly reflected the rate of positive versus negative views on specific COVID-19 vaccine-related content, expressed as either likes vs. dislikes or supporting vs. disapproving content. Analysis of TikTok videos revealed a similar distribution of videos both encouraging/discouraging COVID-19 immunization, 87 and similar results were obtained for YouTube as well. 88 On the other hand, an analysis of Tik-Tok videos revealed that the vaccine produced by Moderna gained considerably greater portion of positive sentiments compared with the one produced by Pfizer (56.8% vs 20.6%, respectively). 89 It is encouraging that videos produced by health experts obtained a greater number of likes, compared with those posted by media. 88 Studies that analyzed the tone from Twitter posts report that in December 2020, the rate of negative sentiment towards COVID-19 immunization was 14% (with 34% neutral and the rest positive), 90 while another study observed around 30% negative sentiment in posts analyzed in December 2020, however, it focused on posts published in the Spanish language under the hashtag #yonomevacunoand only focused on the movement against vaccination passports. 56 Studies that were geo-located observed that among the Indian Twitter users, almost half of the posts were in a neutral tone with only 17% being classified as negative, 91 while another study that focused on posts regarding vaccinerelated side effects identified around 20% were negatively-oriented and more than 30% were positive, implying that nearly 80% of the tweets were either positive or neutral. 92 On the other hand, Indonesian Twitter users seem to express mostly negative attitudes towards the current immunization campaigns, with 75% of posts being classified as having a negative opinion. 93 In Bangladesh, Facebook users mostly reacted to vaccinerelated posts positively. 94 Interestingly, the Iranian Twitter users reported similar rates of positive and negative emotions towards foreign and homegrown vaccines explored between April and September 2021 (43 vs 40% were positive and 45 vs 40% were negative, for foreign and domestic vaccines, respectively). 95 The USA general public sentiment showed a decrease in negative sentiment during the first two months after starting the vaccination (after December 2020) with a total of one-third negatively oriented tweets. 96 Another study on USA users found that they expressed similar rates of positive and neutral sentiment towards COVID-19 vaccination in February/March 2021 (the portion of those expressing negative sentiment was the lowest). 97 Finally, in July 2021 the UK Twitter users expressed similar rates of negative and positive sentiments towards current immunization campaign, as shown by the portion of positive and negative tweets obtained from the study period. 98 Studies reporting fluctuation trends of vaccine sentiments on social networks caused by some driver events Nine studies reported how social media vaccine sentiments varied depending on some key events ( Table 4 ). Events that were identified as those that triggered an increase in the positive emotions towards vaccination were: the announcement about the vaccines' effectiveness, 99−102 the arrival of the vaccines in UK hospitals, 99 the announcement of the first human vaccine trial (UK Twitter and Facebook users) and Donald Trump's announcement regarding a vaccine being ready in a few weeks (US Twitter and Facebook users), 100 as well as the decrease in number of positive COVID-19 cases in Korea. 50 Some of the events that were classified as stimulating negative discussions surrounding COVID-19 vaccines were: popularity of conspiracy theories (related to Bill Gates and microchips), 103 the authorization in the UK of the Pfizer BioNTech COVID19 vaccine, 99 the UK opting out of the European Union vaccination scheme and halting of the phase III vaccine trials at the University of Oxford owing to safety concerns (documented among the UK and USA users), 100 the growth in the occurrence of "fake news" and "misinformation" on social media, 100 the increase in the number of COVID-19 infected cases in Korea (as observed among Korean Twitter users). 50 Finally, Table 5 presents a synthetic summary the main findings of each category of studies included in March, the number of positive tweets temporarily increased in comparison to other days. The tweets posted on 15 March were related to keywords such as "relief" and "experience". this systematic review, in addition to their implications for public health policy, which is discussed further in the discussion section below. This systematic review of the literature sought to explore the diverse role social media can play in understanding, as well as shaping, attitudes toward COVID-19 vaccination among a variety of populations. The data from cross-sectional studies that explored the associations between social media use and vaccination intentions were mainly driven from US-based populations, followed by Chinese-based ones. Among the studies that used different data mining approaches for investigating public opinion related to COVID-19 vaccines, less than half of them (40%) were geo-specific and analyzed posts from users across the world. The rest of these studies did not limit their search to any geographical location. In our analysis, we identified that most eligible studies used Twitter as the main social media platform for gathering information about COVID-19 vaccine attitudes and opinions. Twitter has been recognized as a social media platform that enables the capturing of real-time data, as one study reported a similarity between 2018 vaccine hesitancy survey data and data obtained from Twitter. 104 The study even suggested that it can be used as a substitute for surveys where such purpose is justified. 104 Another study identified Twitter as a "sentinel tool" which can be used for exploring vaccination public opinion specifically, 105 which additionally justifies its frequent use in the studies included in our systematic review that are exploring the ongoing COVID-19 immunization attitudes and opinions. The studies that investigated the associations between social media utilization and vaccine intentions mainly observed a negative relationship. However, there was also a group of studies reporting a positive association. This positive relationship was confirmed among the young population, which is inconsistent with previous literature that reported younger age groups were more hesitant towards vaccination. Consequently, it warrants future research to explore the potential differences in the pattern of social media usage among age groups. Additionally, it illustrates the need to stratify different age groups when studying the effect of social media on public health practices. Another interesting finding from our qualitative synthesis is that social media is positively associated with vaccine acceptance in some specific groups of people with chronic health conditions (such as AIDS/HIV or neurological disorders). This may be a noteworthy finding suggesting that people with adverse health conditions may be less vaccine hesitant in general, and that this group of people is more open to utilizing social media for deriving their health-related decisions. Policy makers aiming at increasing the vaccine uptake may consider this group of people as a target group when designing their strategies. Furthermore, other studies confirming the positive association enlisted doctors as influencers or the type of content the participants have been exposed to, thus these additional covariates may have shaped the results. Only one study explored the impact of specific professionals as influencers on people's intentions to vaccinate, and this issue deserves further investigation. 106 Emerging research has been documenting a clear role for the "public physician" (e.g., a physician who is "outwardfacing" and is a face of medical authority on social media). Social media is an obvious tool for this engagement and can allow experts to debate topics openly and visibly, potentially identifying false information in real time. 107 The research is still emerging regarding the specific role of a public physician on social media in regard to COVID-19 vaccination. However, this is an important finding and is consistent with research demonstrating the role of doctors on social media in significantly promoting the flu vaccine, including among hard-to-reach populations. 108, 109 Another important point derived from this group of studies is the impact of specific social media on vaccine acceptance/hesitancy. Only one survey explored this issue and observed different acceptance rates among YouTube, Facebook and Twitter users. 42 The difference between such users may stem from the different content they have been exposed to on the specific social media platform, further implying that social media has a great challenge when it comes to combating and addressing COVID-19 related misinformation. Finally, it is important to note that while the studies aid in exploring an association, inferring causal relationships is difficult given the nature of the studies and hence the findings must be interpreted within such context. In this systematic review, we identified one group of studies that explored topics discussed on social media related to views on vaccination. Overall, they reported domination of vaccine hesitant topics, and this domination remained stable over time after analyzing the studies that analyzed the data posted in different time points. This pattern was observed regardless of geographical context of participants. Even after performing the updated search in March 2022 and including a large number of recently published studies, we were able to observe that vaccine-hesitant themes are largely universal and repetitive across different geographical locations throughout different time-points. According to some studies, vaccine-related topics fluctuate over time and their popularity depends on the ongoing news published by the media and on social media, and it is interesting how topics that support immunization are mainly time-dependent while those against are mainly stable over time. Analyses of reasons relating to hesitancy included concerns regarding side effects, distrust in experts, and the vaccine development process and were intertwined with conspiracy theories and unreliable information without solid scientific evidence. These reasons for hesitancy are in line with previous research in the literature that did not utilize social media to analyze reasons for hesitancy, which demonstrates a consistency in hesitancy reasons despite expression on social media or not. Furthermore, some studies reported users expressing concern over vaccination price (China), vaccine supply (Mumbai, São Paulo, and Beijing), as well as vaccine distribution and supply (New York and London). These findings reiterate the notion that not all limitations regarding vaccination are merely related to hesitancy, and that lack of vaccination across the globe is not a homogenous issue. Consequently, public health policy targeting vaccination should be tailored to the specific concerns of the geographical context, and social media has the potential to demonstrate specific challenges that may be targeted across different countries. These findings resemble a strong call for addressing the social and cultural context of the specific population that could be affected by intervention measures. Tailoring intervention measures to the specific context of the population, which is shown to be unique for different geographical contexts, has the potential to enhance the outcome desired from an intervention. The group of studies that focused on distinguishing between anti-and pro-vaccination online communities mainly suggests that the clusters against vaccination are better positioned to spread information than those in favor. Indeed, data derived from Turkish pro-and antivaccine networks pointed to a considerable increase in the anti-vaccination cluster after the pandemic began. One of the proposed problems might be the lack of activity of the medical community on social media in combating the spread of misinformation and providing sound scientific evidence surrounding current immunization programs. Another may include the strategies employed by the opposing communities -the pro-vaccination community are described as using mockery and disrespectful tones against anti-vaxxers and trying to attract new followers by pointing to the previous successes of immunizations; the anti-vaccination community was identified as spreading hysteria with the use of misinformation and conspiracy theories, while they simultaneously found ways to connect with a broader group of people by attracting new followers with other topics (such as alternative medical solutions). According to the current available studies, this made anti-vax clusters better positioned and connected not only among their community, but also within the clusters that have not yet decided which path to follow. With respect to some specific influential figures, one study identified Donald Trump as spreading the most vaccine misinformation on Twitter, while another study that explored this question in pro-vaccine network found that healthcare professionals still comprise a very low portion of active users that are involved in disseminating information related to COVID-19 immunizations. These observations point to the lack of online involvement of medically trained personnel in pro-vaccine clusters. Another speculated reason for why anti-vaccination opinions is more prevalent is the "echo chambers" phenomena, where individuals gather and are surrounded by like-minded people in terms of political and ideological orientation. In turn, this allows opinion-reinforcing information and the creation of polarization on certain health topics, such as vaccination in this case. Previous research has demonstrated the role of such echo chambers, 21, 54, 110 where individuals aggregate and amplify opinions in dispersing misinformation about infectious disease. 111 Indeed one study included in our review, observed that belief echoes can actually be quite strong and impact adult Twitter users' vaccination intention regardless of their education level. 85 These findings are critical for shaping public health policy, especially in times of crises, to ensure there is proper science communication that is perceived as authoritative and to prevent misinformation on social media platforms. These findings can significantly aid in directing intervention measures to minimize misinformation and utilize social media as a platform for understanding behaviors and perceptions. Studies included in this review attempted to analyze the type and/or tone of the content present on social media regarding vaccination. In general, studies observed dissimilar degree of positive and negative emotions expressed by social media users originating from different locations, and their emotions were not constant throughout time. One study points to the fact that the public sentiment depend on the vaccine manufacturer as well. 89 These findings, combined with our analyses of anti-vaccination discussions receiving far more attention on social media, are essential for public health intervention. Firstly, these findings highlight that the issue is not merely an absence of promoting positive public health content on social media. Rather, polarized views against vaccination receive more attention from users and are promoted by heated discussions and echo chambers, as discussed earlier. Consequently, when designing intervention measures, it may be more beneficial to target specific anti-vaccine campaigns and misinformation rather than solely increasing the number of vaccine-encouraging content on social media. The results of studies that investigated how the public vaccine-related sentiment changed after some specific events provided valuable insights into this significant cofactor that inevitably affects and shapes the vaccination intentions of individuals. Specifically, the positively stimulating events included those that announced some significant positive milestones in the vaccine development or distribution, while those that increased the rate of negative sentiments were concentrated on the emerging occurrence of misinformation/ conspiracy theories on the social networks, or reported negative points related to vaccine development or immunization schemes. These findings are crucial for designing successful public health intervention for a multitude of reasons. First, social media provides an opportunity to evaluate and understand public sentiment about vaccinations in real-time, in a quick and efficient manner. This is supported by the huge dependence on social media across the globe, allowing it to be a medium for people to express their emotions and opinions. Consequently, this allows governments to promote COVID-19 vaccinations in a rational and orderly manner. A thorough understanding of how specific events trigger people's emotions can be crucial for governments and public health officials to identify appropriate times of intervention to maximize benefit and guarantee the population's cooperation and engagement. Similarly, it allows for an understanding of when it may not be appropriate to implement specific interventions over others. These results constitute a strong call for policy-makers to adjust vaccination policies rapidly in response to the public health events to promote massive vaccination via dynamic monitoring of public sentiment. 112 Finally, in this systematic review we also observed that data mining and natural language processing techniques proved to be very useful and beneficial in analyzing and understanding public perspectives regarding this critical public health issue and thereby providing meaningful and valuable insights. This is also in line with more emerging research in the literature that was not within the scope of this review, but nevertheless reiterating such notions. 113, 114 The value of observations gained from these studies further justifies its widespread application in different fields including marketing, politics and, in this case, public opinion and public health. 115, 116 Overall, misinformation and the spread of anti-vaccination communities on social media platforms contributes to masking healthy behaviors and promotes practices that contribute to negative health outcomes. While restricting information spread on social media continues to be a complex issue, the findings of this review demonstrate the significant importance of building platforms for disseminating authentic public health messages. This include intervention measures to facilitate communication by figures perceived to be authoritative for the said topic (e.g., doctors, health experts). This review provides a high level of evidence by thoroughly and systematically summarizing the literature regarding the associations between social media and COVID-19 vaccination. Additionally, this review seeks to exemplify the potential that social media has as a platform for public health intervention and a source of information that aids in directing policymaking to address vaccine hesitancy and further promote vaccination uptake worldwide. These issues are exceedingly important in the context of the COVID-19 pandemic, where population-level efforts are extremely necessary in reaching herd immunity and hence alleviating the public health burden of COVID-19. We suggest future studies to explore the impact of specific social media sources (for example, YouTube vs. Facebook), explore temporal trends in social media influence on vaccination attitudes (e.g., especially in relation to vaccine rollout), the role of specific types of professionals as influencers (such as doctors, politicians, celebrities, etc.), or the effects of different types of content on vaccination attitudes in more depth. To the best of our knowledge, this review is one of the first studies to review the literature on a very emerging topic in a time where interventions promoting vaccination uptake are indispensable to our progress against the pandemic Despite those strengths, this review is not without limitations that must be acknowledged. Firstly, this review included several cross-sectional studies. While all cross-sectional studies were assessed for quality, the inability to establish causation is a limitation with all cross-sectional studies. Additionally, and given that such studies utilized self-reported measures to collect data, additional limitations include selection bias, social desirability bias, as well as recall bias. Due to the wide range of studies included in this review, no meta-analysis was conducted, limiting this study with the inability to provide a quantitative synthesis of results. This review sought to capture the current evidence present in the literature regarding the role social media platforms can play in affecting attitudes and behaviors relating to COVID-19 vaccination. This is done through a variety of approaches, including misinformation as well as establishment of communities and campaigns to promote behaviors that are not in line with public health recommendations globally. Given the high public health burden of COVID-19 as well as the current limitations in reaching herd immunity partially due to vaccine hesitancy, public health interventions utilizing social media can be effective in promoting vaccine uptake. All authors contributed to revise work for important intellectual content, gave the final approval of the version to be published, and agreed on all aspects of the work, especially concerning its accuracy and integrity. Further specific activities have been distributed as follows: F.C. conceived the research hypothesis. F.C. and A.P. designed the study. A.P. and Y.A.A. performed the article screening. A.P., Y.A.A., G.F., V.P., A.M. and A.L. performed the data extraction and the quality assessment. All authors had access to data and verified it (F. C., A.P., Y.A.A., G.F., V.P., A.M., A.L., W.R.). F.C. and W.R. has shaped the manuscript with input from the entire team (written contributions of single paragraphs). All data gathered for qualitative synthesis within this review are available with publication as supplementary files. No restrictions to access, neither investigator support, nor a data access agreement are required. None. 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Supplementary material associated with this article can be found in the online version at doi:10.1016/j. eclinm.2022.101454.