key: cord-0995214-pmj4sao7
authors: Paredes Mogica, J. A.; Nava, V.; Torres, J.
title: COVID-19 Related Mortality: Is the BCG Vaccine Truly Effective?
date: 2020-05-06
journal: nan
DOI: 10.1101/2020.05.01.20087411
sha: 53d98d208d9ba36fdb31b4fd5cf66d076b71e70a
doc_id: 995214
cord_uid: pmj4sao7

The coronavirus disease 2019 (COVID-19) pandemic has become a worldwide emergency. In the attempt to search for interventions that would improve outcomes, some studies have looked at the potential benefit of BCG vaccination. These past studies have found a statistically significant reduction in COVID-19 related mortality in countries with a current universal bacille Calmette-Guerin (BCG) vaccination policy. However, just as the authors themselves noted, the nature of ecological studies make them very prone to the presence of several confounders. This paper took into account demographic differences, economic differences and the different stages of the pandemic in each country; gathering data from publicly available sources. It was found that no statistically significant difference exists in mortality rates between countries with a current or prior BCG vaccination policy when compared to those that never had such a program. Nevertheless, the immunostimulatory potential of the BCG vaccine might still prove useful in the development of future vaccines or other prophylactic measures.

The rapid spread of coronavirus disease 2019 (COVID- 19) around the world has become a worldwide emergency. The high mortality of the disease in certain populations, along with the lack of vaccinations or effective antiviral treatment have resulted in significant efforts in trying to find interventions that would improve outcomes .1,2 Previous studies have shown an inverse . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. . https://doi.org/10.1101/2020.05.01.20087411 doi: medRxiv preprint relationship between the bacille Calmette-Guérin (BCG) vaccination policy of countries and COVID-19 related mortality. Countries with a current universal BCG vaccination policy seem to have significantly lower mortality rates when compared to countries that do not implement BCG in their childhood vaccination schedules. 3, 4, 5 The immunostimulant properties of the vaccine could in theory improve outcomes through heterologous immunity. 6 However, just as the World Health Organization noted, the nature of ecological studies make them prone to the presence of several confounders. 7 In this study we aim to take into account the possible confounders when analyzing the difference in mortality rates between countries with and without history of a universal BCG vaccination program.

The bacille Calmette-Guérin (BCG) vaccine has been around since the early XX century and it still is one of the most widely used vaccinations around the world. Obtained by attenuating strains of Mycobacterium bovis, it is named after Calmette and Guérin and it was first used in a human subject in France, 1921. 8,9 As of today, it is part of childhood vaccination programs in countries where the prevalence of tuberculosis (TB) is high. The BCG vaccine provides a protective effect against tuberculous meningitis and miliary disease in children; however, it doesn't protect against primary infection or reactivation of latent pulmonary infection. 10, 11 The variable effectiveness of the vaccine in adults has led to different vaccination policies around the world. Countries such as the United States do not recommend its use due to the low risk of infection by Mycobacterium tuberculosis (Mtb) and its potential for interference with tuberculin test reactivity. 11 Similarly, in some regions around the world such as the United . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. . https://doi.org/10.1101/2020.05.01.20087411 doi: medRxiv preprint Kingdom (UK), the BCG vaccine used to be part of the national vaccination program but is no longer recommended for routine use. In contrast, the majority of other countries around the world, such as most Latin American countries, still have a universal vaccination program. 12, 13 Although often considered interchangeable, the existence of different strains of the vaccine and their respective use among countries, also serve as a potential confounder to its variable efficacy. 14 The safety record of BCG, along with its immunostimulatory potential, have proven useful for its application in other fields such as in the management of cancer and autoimmune diseases. Used in conjunction with transurethral resection, intravesical instillation of BCG is currently the standard of care for non-muscle invasive bladder cancer. 15, 16 The mechanism of action in this setting involves innate and adaptive immune responses; after BCG is internalized in the damaged urothelium, it promotes the recruitment of leukocytes by stimulating the release of proinflammatory cytokines (e.g. FN-α, IFN-γ, IL-2, IL-12). 16, 17, 18 Furthermore, BCG has also been used in the management of melanoma and has even been reported to reduce the risk of developing childhood leukemia. 19, 20, 21 Regarding BCG use in autoimmune diseases, some evidence has been found that it might positively affect type 1 diabetes mellitus and multiple sclerosis. 22 The proposed mechanisms in these circumstances are BCG induced up-regulation of the glycolytic pathway and the induction of apoptosis in activated autoreactive T-cells. 23, 24 The properties of the BCG vaccine mentioned earlier (i.e. good safety record and immunostimulatory potential), in addition to its low production cost and it being temperature stable, prove useful when researching recombinant technology. 22 The lack of efficacy to protect adults against TB infection, has led to significant efforts to improve the current BCG.

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The cross-protection the immune system confers between two unrelated pathogens is termed heterologous immunity, and it is an established phenomenon observed in BCG vaccination. Two main mechanisms have been proposed to explain this: cross-protection by heterologous responses of memory T cells and trained immunity. 6 T cell cross-reactivity is the result of antigens of two different pathogens reacting with the same pool of memory T cells, producing an adaptive immune response. 30 On the other hand there is trained immunity, a term that was coined by Netea et al. in 2011. 31 It refers to the enhanced response of the innate immune system to a secondary infection, after exposure to a primary pathogen. This process has been suggested to be the result of epigenetic reprogramming of monocytes, macrophages and natural killer (NK) cells. 32,33 These epigenetic changes, mostly through histone modifications, result in an increased ability of these cells to secrete proinflammatory cytokines; especially IFN-γ, TNF and IL-1β. 34 Also of note, is the shift these cells undergo from oxidative phosphorylation to aerobic glycolysis, which is thought to impact the induction of trained immunity as well. 35

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The copyright holder for this preprint this version posted May 6, 2020. . https://doi.org/10.1101/2020.05.01.20087411 doi: medRxiv preprint 5 The previous findings seem to explain the BCG-associated reduction in all cause mortality within the first years of life in some observational studies. 36, 37 This reduction in mortality has been found to be more noticeable in regions with high childhood mortality and infectious disease burden such as West Africa. 38, 39 Similarly, the rates of infection by non mycobacterial pathogens, were found to be lower in children vaccinated with BCG. 40 In a study conducted by Hollm et al.

in 2014, more than 150,000 children in 33 different countries were found to have a risk reduction of 17% to 37% for the development of acute lower respiratory infections. 41 However, some studies such as the Danish Calmette Study have found no differences in rates of infection or hospitalization in children up to the age of 15 months; partially explained by the lower infection disease burden in the region. 42 The protective effect seems to wane over time and some studies have looked at the potential benefits for revaccination in the elderly. 40 China. 44, 45 It belongs to the family of coronaviruses, which include severe acute respiratory syndrome-CoV (SARS-CoV) and Middle East respiratory syndrome-CoV (MERS-CoV). The pandemic has spread to 6 continents and is estimated to have infected over 2.1 million people and caused over 145,000 deaths worldwide as of April 16, 2020. 46 The population over 45 years . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. . https://doi.org/10.1101/2020.05.01.20087411 doi: medRxiv preprint 6 old is more at risk of developing complications and the risk rises with increasing age; the population over 45 and over 65 years old respectively account for 95% and 72% of all COVID-19 related deaths in New York City. 47, 48 The rapid spread of the disease, its high mortality rate in certain populations (e.g. 7.2% to 8.9%

in patients aged 70 to 79 years old) and, the lack of vaccines or effective antiviral treatment, has made this a worldwide emergency. 1,2 Significant efforts have been made in order to develop strategies for improving outcomes. Of particular interest to some researchers, is the surprisingly low number of cases and mortality rates in countries who currently have a universal BCG vaccination program. Indeed, a negative association was found between BCG vaccination and mortality rates, which could be explained through heterologous immunity. Of note, several confounders were identified by the authors themselves; under-reporting, SARS-CoV-2 capability testing and differing lockdown measures all could contribute to the apparent lower mortality rate. 3, 4, 5 Methods Countries with at least 1,000 COVID-19 cases and at least 1 million inhabitants were included in the study. To adjust for differences in the epidemic time curve, the number of deaths at the time the country hit the case number 1,000 were also compared. Countries were divided in three main groups; countries with no history of universal BCG vaccination policy, countries with an actual universal vaccination policy and countries with prior BCG vaccination programs that would have resulted in the vaccination of the adult and elderly population. Countries that had a universal . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. . https://doi.org/10.1101/2020.05.01.20087411 doi: medRxiv preprint 7 BCG vaccination policy introduced in 1955 or earlier that lasted until at least 1975 were included. Countries with no data available were excluded from the study.

To avoid any possible bias on each country's capability of testing and resources available for healthcare, only high income countries were compared. As per The World Bank classification, high income countries were defined as having a Gross National Income (GNI) per capita higher than 12,375 US dollars. 49 Data on BCG vaccination policies was retrieved from The BCG World Atlas. 50 Total population and GDP income data was retrieved from The World Bank Data. 51 The number of COVID-19 cases and COVID-19 attributable mortality were retrieved from the Worldometer website. 46 The IBM SPSS program was used to analyze data by using the t-test and analysis of variance (ANOVA) test.

After analysis, 20 countries met the inclusion criteria for the study. Out of these, 5 high income countries never had a universal BCG vaccination policy (table 1) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

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The copyright holder for this preprint this version posted May 6, 2020. The mean number of deaths at the 1000th case for countries with current or previous BCG vaccination programs and countries that never implemented BCG vaccination in their schedules was 15.7 ± 17.5 and 20.2 ± 9.2 respectively. No statistically significant difference was noted . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. . https://doi.org/10.1101/2020.05.01.20087411 doi: medRxiv preprint was 18.7 ± 3.6; Japan (28.2%), Israel (11.3%) and Chile (11.8%) were more than 1.5 SD's away from the mean. When comparing the number of deaths at the 1000th case and the total number of deaths per 1 million population of the three groups, no statistically significant difference was found when these countries were excluded from the analysis (p = > 0.05).

The results found in this study seem to suggest no statistically significant difference in mortality arises from BCG vaccination. This is in contrast to previous studies that found an inverse relationship between mortality rates and BCG vaccination status. 3, 4, 5 A striking difference in deaths per 1 million population was noted between current BCG using countries and countries that never implemented a universal vaccination program, which is similar to the results found in previous studies. However, just as the WHO noted, this analysis doesn't reflect demographic differences, disease burden and the different stages of the pandemic in these countries. 7

When trying to account for confounders by comparing the number of deaths at a particular point in time and considering demographic differences, no statistically significant difference was found. Furthermore, the number of deaths at the 1000th case were actually lower in countries that had a prior BCG vaccination program (9 ± 9.4) when compared to countries with an actual universal vaccination policy (29.2 ± 23.1). Nevertheless, this finding was brought into question as no difference was found when accounting for demographic differences in these countries.

These results seem to fit in line with previous findings regarding BCG-induced heterologous immunity. Findings seem to suggest an increase in immune response, however this is highly . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. . https://doi.org/10.1101/2020.05.01.20087411 doi: medRxiv preprint dependent on the time the research subjects received the BCG vaccine. The population that has been previously shown to benefit from BCG-induced heterologous immunity is restricted to the pediatric population. [36] [37] [38] [39] [40] [41] [52] [53] Furthermore, this benefit seems to only be apparent when the regions involved already have high childhood mortality, as evidenced by the Danish Calmette Study. 42 The pediatric population can contract COVID-19 and even serve as potential carriers because of their mostly asymptomatic course. 54 Although childhood vaccination seems to have no effect on mortality rates of the adult population, the BCG vaccine could still prove useful. The early genome sequencing of SARS-CoV-2 is helping current efforts in developing a vaccine against it. The use of recombinant technology could help with the early development of a vaccine by using vectors and embedding them with SARS-CoV-2 antigens. 56 As stated before, rBGC vaccines could help in this field as they induce a considerable immune response and have been used before as vectors to nonmycobacterial related antigens. 26-28 Also of interest is the potential effect revaccination might have on the elderly population. The potential of BCG-induced heterologous immunity after revaccination has shown promise, as demonstrated by Warhana et al. 43 Two current clinical trials are looking into this by vaccinating healthcare workers with BCG to see if there is any potential benefit. 57, 58 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. There are some limitations to be considered in this study. Although great care was taken to try to avoid as many confounders as possible, some still remain. The difference in mortality seems to be higher in the male population than in the female population, which could affect the overall mortality countries with differing demographics. 59 Also, some populations are more genetically susceptible to certain diseases and complications than others (e.g. Latin Americans and diabetes). 60 Some researchers are looking into this by analyzing genetic differences in the expression of the ACE2 receptor or the CCL2 and MBL genes, which could affect susceptibility to SARS-CoV-2. 3, 61, 62 Finally, each country's response to the outbreak affects subsequent policies taken by other countries and although the coverage of countries currently using BCG was mostly over 90% no data was available for countries with prior vaccination programs. 50

The status of BCG vaccination policy does not seem to have an effect in COVID-19 related mortality. Although data is difficult to analyze due to the presence of multiple confounders, statistically significant differences seem to disappear once much of the confounders are taken into consideration. Nevertheless, the immunostimulant properties of the BCG vaccine could prove useful in future developments of a vaccine using recombinant technology. Likewise, revaccination with BCG has shown promise in the past for the induction of heterologous immunity. Current medical trials are underway looking at the potential benefits of vaccinating healthcare workers.

No conflicts of interest to disclose.

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The copyright holder for this preprint this version posted May 6, 2020. is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. . https://doi.org/10.1101/2020.05.01.20087411 doi: medRxiv preprint

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The copyright holder for this preprint this version posted May 6, 2020. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted May 6, 2020. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

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