Stop the super-spreaders!

Great NYT Op-Ed on the central role super-spreaders and super-spreading events are playing in the pandemic.  It really is amazing the proportion of the overall transmissions that seem to be related to super-spreaders.  We cannot know which people are especially infectious.  But we can and do know what types of events (church services, crowded bars and night clubs) are especially likely to lead to super-spreading events.  So, let’s make damn well sure we keep those from happening.  I would’ve liked to have seen an estimate on what just eliminating super-spreading events does to Rt, but would not be surprised if it puts it below 1.0.  Anyway…

In the case of SARS-CoV-2, evidence is growing that superspreading is a hugely significant factor of total transmission.

Take Hong Kong, which as of June 2 had 1,088 confirmed or probable cases (and four deaths), for a population of about 7.5 million. The city has managed to largely suppress local outbreaks of Covid-19 without a lockdown or mandatory blanket stay-at-home orders, favoring instead a strategy of testing people suspected of being infected, tracing and quarantining their contacts and isolating confirmed cases in the hospital — coupled with outright bans or other restrictions on large social gatherings.

After these measures were progressively relaxed in recent weeks, a new outbreak of seven cases, possibly a superspreading event, has been reported over the past few days: Three are employees of a food-packing company; the other four live in the same housing estate as one of the employees.

We recently published a preprint (a preliminary paper, still to be peer-reviewed) about 1,038 cases of SARS-CoV-2 in Hong Kong between Jan. 23 and April 28 that, using contact-tracing data, identified all local clusters of infection.

We found that superspreading has overwhelmingly contributed to the transmission of SARS-CoV-2 in the city overall…

In our study, just 20 percent of cases, all of them involving social gatherings, accounted for an astonishing 80 percent of transmissions. (That, along with other things, suggests that the dispersion factor, k, of SARS-CoV-2 is about 0.45).

Another 10 percent of cases accounted for the remaining 20 percent of transmissions — with each of these infected people on average spreading the virus to only one other person, maybe two people. This mostly occurred within households.

No less astonishing was this corollary finding: Seventy percent of the people infected did not pass on the virus to anyone… [emphases mine]

It stands to reason, too, that a highly contagious person is more likely to spread the infection in a crowd (at a wedding, in a bar, during a sporting event) than in a small group (within their household), and when contact is extensive or repeated.

Transmission is more likely during gatherings indoors than outdoorsSimply ventilating a room can help. We believe that with the South Korean call-center cluster, the essential factor of transmission was the extent of time spent in a crowded office area.

Also consider this counterexample: Japan. The government recently lifted a state of emergency after controlling its epidemic without having put in place any stringent social distancing measures or even doing much testing. Instead, it relied on largely voluntary measures encouraging people to stay at home and advice to avoid overcrowding in public venues.

Will also mention here that Japan also adopted very widespread mask use.

But the considerable role of superspreading in this pandemic should be reassuring, too, because it also suggests a way to stop SARS-CoV-2 that is both less onerous and more effective than many of the strategies that have been pursued so far.

The epidemic’s growth can be controlled with tactics far less disruptive, socially and economically, than the extended lockdowns or other extreme forms of social distancing that much of the world has experienced over the past few months.

Forget about maintaining — or, if infections resurge, resuming — sweeping measures designed to stem the virus’s spread in all forms. Just focus on stopping the superspreading.

Of course, what remains to be well understood is whether a K-12 school is a likely source of super-spreading.  Evidence so far, actually, suggests generally not.  Or, what about a socially-distanced university classroom (as NC State and many others are currently planning)?

Anyway, overall, I do find this quite encouraging because I would be so willing to give up all the known super-spreading if I did know that in so doing my kids could go to school, I could teach my socially-distanced classes, and socialize with a small number of friends.

Also, I think that the lack of predicted surges/waves in states that opened up early, e.g., Georgia, or were always pretty lax, e.g., Texas, might well be attributed to the fact that even with their “opening up” there’s a lot less of the typical super-spreading type events.

For now, I’ll leave you with this once again.

COVID-19 Information and Resouces

About Steve Greene
Professor of Political Science at NC State

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