Have we learned anything new recently about COVID-19 transmission?

Infection and Spread

A: Recent analysis of infection clusters in Japan bolsters evidence that heavy breathing in close proximity is high risk activity for community spread.

Japan has been aggressively tracing “clusters” of coronavirus infections, and the recently summary published by the CDC (linked below) is full of interesting findings. “Clusters” were defined as 5 or more confirmed cases with primary exposure reported at a common event or venue. “Primary cases” were those believed to be the original source of infection in a cluster.

-Health care facilities and nursing homes were an important source of infection clusters, accounting for 46% of the total 61 clusters examined.

-Many community clusters were associated with heavy breathing in close proximity, such as singing at karaoke parties, conversations at clubs and bars, and exercising in gyms.

-The largest non-healthcare cluster was among more than 30 people who attended a live music concert, including performers, audience members, and event staff.

-The most common age group for probable primary cases were 20-39 year olds (see picture).

-Where date of transmission could be determined, 41% of the primary cases were presymptomatic or asymptomatic at the time of transmission, and only 1 had a cough at the time of transmission.

Some caveats: In this report, only 61% of cases of local transmission had links to known cases and thus could be traced in this way. This means 39% of cases came from unknown sources, so activities not mentioned here can still carry risk.

Overall this new evidence supports Japan’s push for the population to avoid the “Three Cs”: closed spaces with poor ventilation, crowded places, and close-contact settings. Sadly, events with lots of cheering and singing are likely to be late on the list of activities to which we can safely return.

The continued evidence of presymptomatic and asymptomatic spread also underscores the benefits of masking, since many will not know they are sick or at risk of spreading. This is even more true for young adults, who may be more at risk of spreading due to their social contacts and lower likelihood of symptoms.

So MASK UP friends, and save the Karaoke for home for now!

CDC article on Japan Clusters

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