What is a superspreader event for COVID-19 ?
Text updated on 2020-06-23
A situation is superspreading for COVID-19 when there are many people at risk of being infected with SARS-CoV-2 and conditions that favour the likelihood of transmitting the coronavirus: repeated or close contact, the presence of highly infectious people, highly infectious activities such as singing, and environmental conditions that favour the survival of the virus (low temperature and dry atmosphere).
As observed during the SARS and MERS epidemics, the COVID-19 disease only seems to flare up in the presence of an event or situation where many people were simultaneously contaminated by a few individuals, or even just one. This is the case for choirs, slaughterhouses, cruise ships, or military ships.
It is possible to distinguish four types of super-propagating situations:
- biological situations that increase infectivity, e.g., high viral load in the oropharynx or lungs (hypothesis) ;
- behavioural or social situations such as, for example, subjects who, because of their profession, come into contact with a large number of people;
- high-risk establishments such as slaughterhouses, community homes, HPAHs, care facilities, or prisons.
- Opportunistic scenarios which are occasional high-density gatherings (e.g., ships, unventilated public transport), especially those involving shouting or singing (religious services, concerts, nightclubs).
COVID-19 appears to be transmitted mainly in closed environments, poorly ventilated environments, high traffic areas, and when exposures are of long duration (1 hour or more).
Thus, what defines a super-propagative situation is both the presence of many people likely to be infected and conditions that increase the likelihood of being infected (e.g., frequently repeated or close contacts, presence of highly contaminating people or activities).
In order to avoid further exponential growth of the epidemic, it is important to limit super-propagating situations: avoid large gatherings, put on a mask, and respect barrier gestures and rules of social distancing.
Article submitted on May 27th which takes stock of the superspreader events in the context of the spread of the epidemic of COVID-19 and identifies the key factors of a superspreader event.Althouse, B. M., Wenger, E. A., Miller, J. C., Scarpino, S. V., Allard, A., Hébert-Dufresne, L., & Hu, H. (2020). Stochasticity and heterogeneity in the transmission dynamics of SARS-CoV-2. arXiv preprint arXiv:2005.13689.
A Hong Kong resident visited the cruise ship, Diamond Princess, on January 25, 2020 and tested positive. The number of positive cases on the cruise ship rose rapidly to approximately 700 people, or 17% of all passengers in 20 days.Mizumoto, K., Kagaya, K., Zarebski, A., & Chowell, G. (2020). Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020. Eurosurveillance, 25(10), 2000180.
More than 100 cases of people with COVID-19 have been traced to nightclubs in Seoul that were visited by a young man who later tested positive.Kwon, J. (2020). A new coronavirus cluster linked to Seoul nightclubs is fueling homophobia. CBS News. May 12, 2020.
An analysis of more than 3,700 patients infected with SARS-CoV-2 indicates that the amount of coronavirus measured in specimens is highly variable among individuals.Jones, T. C., Mühlemann, B., Veith, T., Biele, G., Zuchowski, M., Hoffmann, J., ... & Drosten, C. (2020). An analysis of SARS-CoV-2 viral load by patient age. medRxiv.
In Japan, many cases of "cluster" contamination have occurred in closed places: a sports hall, restaurant boat, hospital, festival where there were tents with minimal ventilation for eating.Nishiura, H., Oshitani, H., Kobayashi, T., Saito, T., Sunagawa, T., Matsui, T., ... & Suzuki, M. (2020). Closed environments facilitate secondary transmission of coronavirus disease 2019 (COVID-19). medRxiv.
Compilation of 135 cases of COVID-19 contamination: analysis of the table provided by the authors indicates that more than 95% of the cases are indoor transmissions. For the remaining 5%, it is not clear whether transmission occurred outdoors. For example, contamination on construction sites may have occurred in construction huts.Leclerc, Q. J., Fuller, N. M., Knight, L. E., Funk, S., Knight, G. M., & CMMID COVID-19 Working Group. (2020). What settings have been linked to SARS-CoV-2 transmission clusters? Wellcome Open Research, 5(83), 83.
The 60 members of a choir gathered in a room to sing for 2.5 hours on March 10, 2020 in Washington State, USA. They kept a safe distance, used hydro-alcoholic solutions, but did not put on masks. Three weeks later, 45 of them tested positive for COVID-19.Los Angeles Times article of March 29, 2020.
An analysis of 50 clusters in which more than 5 people were infected between January and April 2020 in Japan shows that super-propagating situations are mainly observed in health care facilities, nursing homes, restaurants, bars, workplaces, and music events. Many clusters are associated with heavy breathing: singing at karaoke parties, cheering in clubs, talking in bars, and exercising in gyms.Furuse, Y., Sando, E., Tsuchiya, N., Miyahara, R., Yasuda, I., Ko, Y. K., ... & Nagata, S. Clusters of Coronavirus Disease in Communities, Japan, January-April 2020.
An analysis of the COVID-19 transmissions in 100 countries up to the end of February 2020 suggests that about 80% of transmissions are caused by a small fraction of infectious individuals (about 10%). Note that other studies estimate this fraction to be around 10-20%.Endo, A., Abbott, S., Kucharski, A. J., & Funk, S. (2020). Estimating the overdispersion in COVID-19 transmission using outbreak sizes outside China. Wellcome Open Research, 5(67), 67.