Volume 27, Number 1—January 2021
Superspreading Event of SARS-CoV-2 Infection at a Bar, Ho Chi Minh City, Vietnam
We report a superspreading event of severe acute respiratory syndrome coronavirus 2 infection initiated at a bar in Vietnam with evidence of symptomatic and asymptomatic transmission, based on ministry of health reports, patient interviews, and whole-genome sequence analysis. Crowds in enclosed indoor settings with poor ventilation may be considered at high risk for transmission.
Superspreading events occur when a few persons infect a larger number of secondary persons with whom they have contact (1,2). For severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), an R0 of 2–3 with 6–8 secondary cases has been suggested to constitute a superspreading event (3).
Although SARS-CoV-2 is known to be transmitted through droplets and fomites, there has been growing evidence of airborne transmission (4,5). Better understanding of specific settings in which superspreading events are facilitated remains critical to inform the development and implementation of control measures to avoid future waves of the pandemic (5).
On March 18, 2020, a 43-year old man, patient 1, sought treatment at the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam, for fever, cough, muscle aches, fatigue, and headache. A sample from a nasopharyngeal throat swab specimen taken at admission tested positive for SARS-CoV-2 by reverse transcription PCR.
During the 14 days before the onset of his symptoms on March 17, he had traveled to Thailand and within Vietnam, between Hanoi and Ho Chi Minh City. From 10:00
After the confirmed diagnosis of COVID-19 in patient 1, we used contact tracing and testing to detect 18 additional PCR-confirmed cases. Of these, 12 (patients 2–13) were at bar X during the evening of March 14; the other 6 (patients 14–19) were contacts (Table; Appendix [[ANCHOR###F1######Anchor]]Figure). Of the patients with confirmed cases attending the celebration, 4 were in close contact with patient 1: patients 2–4 went to the celebration with patient 1 and patient 6 worked as a waiter in the bar. Patients 2 and 3, who were roommates, had traveled to Malaysia and returned to Vietnam, patient 2 on March 13 and patient 3 on March 6. The other patients, except for patient 1, had no recent history of travel outside of HCMC (Table).
By exploring the epidemiologic links discovered from in-depth interviews, we identified 3 possible transmission chains involving patients who attended the March 14 celebration (Table; Figure; Appendix Figure). Of these, 2 or 3 patients (patients 5, 10, and possibly 14) were asymptomatic but transmitted SARS-CoV-2 to their contacts (Table; Figure). None of the 19 patients with confirmed cases reported that they had respiratory signs or symptoms on March 14–15. However, in addition to patient 1, a total of 5 others developed mild respiratory symptoms (patient 4 on March 16, patient 6 on March 21, patient 9 on March 25, patient 13 on March 26, and patient 17 on March 27), suggesting an incubation period of 2–12 days. Follow-up data were available for 12 patients who participated in our clinical study (Appendix). Six remained asymptomatic during follow-up (Appendix Table 1).
A total of 11 whole-genome sequences of SARS-CoV-2 were obtained from the patients in the cluster. The obtained sequences were either 100% identical or different from each other by only 1–2 nt (Appendix Table 2). Phylogenetically, they clustered together tightly but were different from sequences obtained from other cases in Ho Chi Minh City during the same period.
As of September 15, 2020, only 30 cases of locally acquired infection had been reported in Ho Chi Minh City (6), but this cluster represents the only documented superspreading event (6,7). Together with data from previous reports (3,8,9), these data suggest that closed settings are facilitators of community transmission of SARS-CoV-2. The mechanism by which infected people without symptoms spread SARS-CoV-2 to others, especially in closed settings, warrants further research, including on transmission through aerosols, which has been suggested (4,10).
The high level of genome sequence similarity between the SARS-CoV-2 genomes obtained from the patients and the tight clustering on the phylogenetic tree strengthen the epidemiologic link between the PCR-confirmed cases from this cluster. Together with contact history, these data also support transmission chains involving asymptomatic carriers (patients 5 and 14) as the sources of the ongoing infection. However, the identity of the patient in the index case from the bar could not be confirmed, in part because in-depth interview data were available from only 8 of 13 patients with confirmed cases who consented to participate in the study. In conclusion, our results emphasize that persons in crowded indoor settings with poor ventilation may be considered to be at high risk for SARS-CoV-2 transmission.
Dr. Chau is the director of the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam. He is a frontline healthcare worker in the COVID-19 pandemic.
We are indebted to Nguyen Thanh Ngoc, Le Kim Thanh, and the OUCRU IT/CTU/Lab management departments, especially Ho Van Hien, Dang Minh Hoang, and Nguyen Than Ha Quyen, for their support. We thank Maia Rabaa at OUCRU for her initial discussion about the analysis, and Leigh Jones at OUCRU for her input with some of the epidemiological data. Finally, we thank the patients for their participation in this study and the doctors and nurses of the Hospital for Tropical Diseases, who cared for the patients and provided the logistic support with the study.
This study was funded by the Wellcome Trust of Great Britain (106680/B/14/Z and 204904/Z/16/Z).
- Wang SX, Li YM, Sun BC, Zhang SW, Zhao WH, Wei MT, et al. The SARS outbreak in a general hospital in Tianjin, China — the case of super-spreader. Epidemiol Infect. 2006;134:786–91.
- Cho SY, Kang J-M, Ha YE, Park GE, Lee JY, Ko J-H, et al. MERS-CoV outbreak following a single patient exposure in an emergency room in South Korea: an epidemiological outbreak study. Lancet. 2016;388:994–1001.
- Adam DC, Wu P, Wong JY, Lau EHY, Tsang TK, Cauchemez S, et al. Clustering and superspreading potential of SARS-CoV-2 infections in Hong Kong. Nat Med. 2020; Epub ahead of print.
- Morawska L, Milton DK. It is time to address airborne transmission of COVID-19. Clin Infect Dis. 2020;
- World Health Organization. Transmission of SARS-CoV-2: implications for infection prevention precautions. 2020. [cited on 2020 Jul 24] https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions
- Ministry of Health of Vietnam. Updated information about COVID-19 pandemic. Official page on acute respiratory infections COVID-19 [cited 2020 Sep 5].http://ncov.moh.gov.vn
- Thanh HN, Van TN, Thu HNT, Van BN, Thanh BD, Thu HPT, et al. Outbreak investigation for COVID-19 in northern Vietnam. Lancet Infect Dis. 2020;20:535–6.
- Pung R, Chiew CJ, Young BE, Chin S, Chen MIC, Clapham HE, et al.; Singapore 2019 Novel Coronavirus Outbreak Research Team. Investigation of three clusters of COVID-19 in Singapore: implications for surveillance and response measures. Lancet. 2020;395:1039–46.
- Ghinai I, Woods S, Ritger KA, McPherson TD, Black SR, Sparrow L, et al. Community Transmission of SARS-CoV-2 at Two Family Gatherings - Chicago, Illinois, February-March 2020. MMWR Morb Mortal Wkly Rep. 2020;69:446–50.
- Somsen GA, van Rijn C, Kooij S, Bem RA, Bonn D. Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission. Lancet Respir Med. 2020;8:658–9.
TableCite This Article
Original Publication Date: October 15, 2020
1Members of the group are listed in the [[ANCHOR###SD1###Appendix###Anchor]].