What to avoid the most when you go back to work?

"You’ve also probably heard of SARS-CoV-2’s R0 (R-naught), or basic reproductive number, the average number of people to whom an infected person passes on a new virus when no measures to contain it have been taken. This coronavirus’s R0 is thought to range between 2 and 3; an epidemic is curbed when that figure drops below 1, the replacement rate.

But that figure has limitations: It doesn’t convey the vast range between how much some infected people transmit the virus and how little others do.

This is why epidemiologists also look at a virus’s dispersion factor, known as “k,” which captures that range and so, too, the potential for superspreading events. To simplify: The fewer the number of cases of infection responsible for all transmissions, the lower k generally is (though other factors, like the R0, also are relevant).

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.

Of the 349 local cases we identified — the remaining 689 cases were imported from other territories — 196 were linked to just six superspreading events. One person alone appears to have infected 73 individuals after frequenting several bars in late March.

Weddings, temples, hot-pot dinners, work parties and karaoke venues featured in the other clusters.

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.

This much, though, is known: The infectiousness of SARS-CoV-2 appears to peak within the first few days of the onset of Covid-19 symptoms and then decrease with time. That said, one can be contagious before displaying symptoms or without ever displaying any symptoms. (Hence the importance of face masks.)

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 outdoors. Simply 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.

But the record in both places, and elsewhere, points to the same conclusion: It’s not just that superspreading events are happening with SARS-CoV-2; they appear to be driving much of the pandemic.

This fact is alarming and reassuring at the same time.

It’s alarming because it suggests a virus swift and efficient, and so seemingly unstoppable.

In essence, Japan adopted an anti-superspreading strategy. The approach was targeted at limiting what some researchers from Tohoku University have called the “three Cs”: closed spaces, crowds and close contacts."