key: cord-0804897-b3f1k32k authors: Noh, Ji Yun; Song, Joon Young; Hyun, Hak Jun; Yoon, Jin Gu; Seong, Hye; Cheong, Hee Jin; Yoon, Soo-Young; Yang, Jeong-Sun; Lee, Joo-Yeon; Kim, Woo Joo title: Risk factors for SARS-CoV-2 transmission in non-household clusters date: 2021-06-08 journal: J Infect DOI: 10.1016/j.jinf.2021.06.003 sha: 893e7d4e3fe7bee35d24806fca98a8ef2f527e0f doc_id: 804897 cord_uid: b3f1k32k nan Ladhani et al. assessed occupational risk factors for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection among staff in care homes. 1 They identified SARS-CoV-2 clusters involving staff; staff working across different care homes had a 3-fold higher risk of SARS-CoV-2 positivity than those who were working in single care homes. In South Korea, three waves of coronavirus disease 2019 (COVID-19) pandemic has been repeated, and each of them was associated with several large clusters in working places, churches and sports centers. 2 In this study, we aimed to evaluate the risk factors for SARS-CoV-2 transmission in non-household clusters of COVID-19 through a thorough epidemiological investigation. From 1 March to 30 August of 2020, we recruited adult patients aged ≥19 years with COVID-19 and their close contacts in the non-household clusters. We defined clusters as two or more confirmed infections with reported close contact. Close contacts were identified through a rigorous epidemiological investigation, including tracking cellular-phone locations, credit-card usage, closed-circuit television images, and in-depth interviews of patients with COVID-19. Those who had been exposed to SARS-CoV-2 in their households or abroad were excluded from this study. In order to evaluate the risk factors of SARS-CoV-2 infection, the SARS-CoV-2-positive group and the SARS-CoV-2-negative group were compared. Participants who were negative for SARS-CoV-2 at the time of a cluster development and were negative for anti-SARS-CoV-2 antibodies in the convalescent phase were included in a SARS-CoV-2-negative group. We found that actions associated with close contact, such as face-to-face conversations, eating together, and using the same toilet as that used by a COVID-19 patient, were significantly more frequent in the SARS-CoV-2-positive group than in the SARS-CoV-2negative group (P < 0.001 for all three variables). Meanwhile, there was no significant difference in the frequencies with which objects were shared with persons with SARS-CoV-2 infection between the SARS-CoV-2-positive and SARS-CoV-2-negative groups (P = 0.779). On multivariable analysis, persons aged ≥ 40 years had a higher risk of SARS-CoV-2 infection than those aged < 40 years (odds ratio [OR]: 5.55), and workers at a call center had 5 a higher risk of SARS-CoV-2 infection than workers at a research building (OR: 14.39). Conversing face-to-face with a COVID-19 patient was revealed to be an independent risk factor for SARS-CoV-2 transmission (OR: 4.11) ( Table 1) . With respect to the age, persons aged ≥ 40 years had a higher risk of SARS-CoV-2 infection than those aged < 40 years. Although case fatality rates of COVID-19 were presented as an age-based exponential increase pattern, the incidence of COVID-19 by age varied according to country. 5 It is possible that the age-related difference in behavioral patterns within a group might affect the risk of SARS-CoV-2 transmission. When clusters were compared according to their location, in this study, the risk of SARS-CoV-2 transmission was found to be significantly higher in a call-center cluster than in a researchbuilding cluster. Call-center workers usually talk continuously when making and receiving phone calls at their workplace. A crowded space and prolonged exposure to respiratory particles from infected patients may facilitate viral transmission. Regarding the transmission mode of SARS-CoV-2, in consistent with our results, a previous study in Singapore showed that verbal interactions were significantly associated with SARS-CoV-2 transmission. 6 The possibility of SARS-CoV-2 transmission by fomites appears to be low in the real world. 7 Notably, five individuals (6.6%) in the SARS-CoV-2-positive group denied that they had participated in face-to-face conversations, eating together, using the same toilets, and sharing objects with patients with COVID-19; these individuals included four from the call center and one from a church. Although close contact events were not identified through investigation, it is uncertain whether these patients might have been infected by the airborne transmission of SARS-CoV-2. Droplets exist across a continuum of sizes, and airflow is crucial in determining the travel distance of droplets. 8 Nonetheless, there is increasing evidence for the airborne transmission of SARS-CoV-2, which demands changes in infectioncontrol measures. 7 Airborne transmission of SARS-CoV-2 can occur in special circumstances, 6 such as those involving enclosed spaces, prolonged exposures to respiratory particles, and inadequate ventilation or air handling. 9 This study has some limitations. First, environmental samples were not taken due to access restrictions when an outbreak occurred. Second, because viral isolates were unavailable, we could not carry out whole-genome sequencing, which is a useful tool to clarify the transmission dynamics in each cluster. In conclusion, this study shows that face-to-face conversations with a patient with COVID-19 was the most significant risk factor for SARS-CoV-2 transmission in non-household clusters. The possibility of fomite-mediated SARS-CoV-2 transmission appears very low in an environment where hand hygiene is emphasized. This study was financially supported by the Korea National Institute of Health, Korea Centers for Disease Control and Prevention (Project number: 2020-ER5314-00). 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