Some Thoughts on the Mid-Term Future of COVID-19

I’m reluctant to engage in any kind of COVID-19 prognostication because figuring out how the human side of the equation will react is, as always, very challenging (and that’s an understatement). That said, I want to follow up on a post I wrote earlier this week, and tie it to an excellent article by Kai Kupferschmidt–and I wish I had seen his article before writing my post. Anyway, to revisit my post about chuckleheads who want to return to the gym*:

First, the reality is that everyone who uses that gym will be fine…until they’re not. As Yogi Berra would have put it had he been an epidemiologist, the best way to avoid getting an infectious disease is to avoid people who are infected. That is, right now, the odds of a suburban area off the beaten path (tourist season hasn’t started yet in Wildwood, NJ) having very many COVID-19 positive people is low. Additionally, a fair number of the infected will be at home (though not the presymptomatic people) and some of the infections will be within a household, so the number of contacts with an infected person will be limited.

That gym will be fine, until someone who is sick decides to use it, at which point there is a non-negligible chance of a local outbreak (like the Provisional IRA used to say, it only has to get lucky once).

At a larger scale, over the next few months, as various locations in the U.S. loose restrictions, what I think we’ll see is a patchwork of local outbreaks, that have a high degree of randomness. Luck will definitely be a player in all of this, as an infected individual happens into an ‘open’ community–and if things break wrong (for the humans, not the virus), then there’s an outbreak. In other words, the frequency is so low and patchily distributed that many people haven’t been exposed to it at all, and won’t be for some random length of time.

What I didn’t add is that this will be like sparks that set off fires: some will self-extinguish immediately, others might burn a small bit and then go out (e.g., one or two people), and some might turn into serious fires (lots of people). But even a massive sweep is really lots of little outbreaks. The reason I thought this–rather inchoately–is because the spread of SARS-CoV2 is itself patchily distributed as Kupferschmidt notes (boldface mine):

But SARS-CoV-2, like two of its cousins, severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), seems especially prone to attacking groups of tightly connected people while sparing others. It’s an encouraging finding, scientists say, because it suggests that restricting gatherings where superspreading is likely to occur will have a major impact on transmission, and that other restrictions—on outdoor activity, for example—might be eased…

Most of the discussion around the spread of SARS-CoV-2 has concentrated on the average number of new infections caused by each patient. Without social distancing, this reproduction number (R) is about three. But in real life, some people infect many others and others don’t spread the disease at all. In fact, the latter is the norm, Lloyd-Smith says: “The consistent pattern is that the most common number is zero. Most people do not transmit.”

…Estimates of k for SARS-CoV-2 vary. In January, Julien Riou and Christian Althaus at the University of Bern simulated the epidemic in China for different combinations of R and k and compared the outcomes with what had actually taken place. They concluded that k for COVID-19 is somewhat higher than for SARS and MERS. That seems about right, says Gabriel Leung, a modeler at the University of Hong Kong. “I don’t think this is quite like SARS or MERS, where we observed very large superspreading clusters,” Leung says. “But we are certainly seeing a lot of concentrated clusters where a small proportion of people are responsible for a large proportion of infections.” But in a recent preprint, Adam Kucharski of LSHTM estimated that k for COVID-19 is as low as 0.1. “Probably about 10% of cases lead to 80% of the spread,” Kucharski says.

That could explain some puzzling aspects of this pandemic, including why the virus did not take off around the world sooner after it emerged in China, and why some very early cases elsewhere—such as one in France in late December 2019, reported on 3 May—apparently failed to ignite a wider outbreak. If k is really 0.1, then most chains of infection die out by themselves and SARS-CoV-2 needs to be introduced undetected into a new country at least four times to have an even chance of establishing itself, Kucharski says. If the Chinese epidemic was a big fire that sent sparks flying around the world, most of the sparks simply fizzled out.

To tie this back to predictions, I think the U.S. will be alright, even with loosened restrictions, over the summer. There will be some localized flare ups, but no massive spread. A gym’s patrons might be hit hard (eventually), but it eventually burn itself out. But when September rolls around and schools and universities are back in session, Memorial Day weekend is over, people get increasingly lax, state to state travel picks up, and workers start returning to offices, all of that mixing among different groups means that one unlucky event can be spread through multiple social circles (e.g., your workplace, your kids’ school, etc.). There will be more sparks and more flammable terrain, and the number of cases will surge.

If we’re not careful, and I haven’t seen much evidence we will be (and the lengthy presymptomatic period of the virus makes it really challenging), then September and October could be really bad, even though the summer won’t be. Though I hope I’m wrong about the fall, obviously.

*Yes, they’re chuckleheads.

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