Thread by @slwirth

A must read to better understand the spreading pattern of COVID-19.
After 9 months of collecting epidemiological data, we know that this is an overdispersed pathogen, meaning that it tends to spread in clusters, but this knowledge hasn’t yet fully entered our preventive practices

https://twitter.com/anil_oezdemir/status/1317430855007174657

“Diseases like the flu are pretty nearly deterministic and R0 (while flawed) paints about the right picture (nearly impossible to stop until there’s a vaccine).” That’s not necessarily the case with super-spreading diseases.

Hitoshi Oshitani of the COVID-19 Cluster Taskforce at Japan’s Ministry of Health and a professor at Tohoku University: Japan focused on the overdispersion impact from early on, likens his country’s approach to looking at a forest and trying to find the clusters, not the trees

He believes the West is getting distracted by the trees, and gets lost among them. To fight a super-spreading disease, policy makers need to figure out why super-spreading happens and need to understand how it affects everything, also contact-tracing methods and testing regimes.

In study after study, we see that super-spreading clusters of COVID-19 almost overwhelmingly occur in poorly ventilated, indoor environments where many people congregate over time— choirs, gyms, restaurants, and such—especially when there is loud talking or singing without masks.

Prolonged contact, poor ventilation, a highly infectious person, and crowding are the key elements for a super-spreader event. These can also occur indoors beyond the 6-feet guideline, as SARS-CoV-2bcan travel through the air and accumulate, especially if ventilation is poor.

Given the huge numbers associated with these clusters, targeting them would be very effective in getting our transmission numbers down.

Overdispersion should also inform our contact-tracing efforts.

Often, we should work backwards to see who first infected the subject.
Because of overdispersion, most people will have been infected by someone who also infected other people, because only a small % of people infect many at a time, whereas most infect zero or maybe one person

BTW this is concurrent with what Drosten said in his latest podcast podcasts.apple.com/ch/podcast/das…

if we can use retrospective contact tracing to find the person who infected our patient, and then trace the forward contacts of the infecting person, we are generally going to find a lot more cases compared with forward-tracing contacts of the infected patient,

which will merely identify potential exposures, many of which will not happen anyway, because most transmission chains die out on their own.

Even in an overdispersed pandemic, it’s not pointless to do forward tracing to be able to warn and test people, if there are extra resources and testing capacity.

But it doesn’t make sense to do forward tracing while not devoting enough resources to backward tracing and finding clusters, which cause so much damage.

Another significant consequence of overdispersion is that it highlights the importance of certain kinds of rapid, cheap tests.

Cheap, low-sensitivity tests are particularly valuable for cluster identification during an overdispersed one.

In an overdispersed regime, identifying transmission events (someone infected someone else) is more important than identifying infected individuals.

Overdispersion also enhances the utility of other aggregate methods, such as wastewater testing, especially in congregate settings like dorms or nursing homes, allowing us to detect clusters without testing everyone.

Once a country has too many outbreaks, it’s almost as if the pandemic switches into “flu mode,”, meaning high, sustained levels of community spread even though a majority of infected people may not be transmitting onward.

Barring truly drastic measures, once in that widespread and elevated mode, COVID-19 can keep spreading because of the sheer number of chains already out there. Plus, the overwhelming numbers may eventually spark more clusters, further worsening the situation.

Could we get back to a much more normal life by focusing on limiting the conditions for super-spreading events, aggressively engaging in cluster-busting, and deploying cheap, rapid mass tests, once we get our case numbers down to low enough numbers to carry out such a strategy?

Many places with low community transmission could start immediately.) Once we look for and see the forest, it becomes easier to find our way out.
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