Super-spreaders, COVID-19, and the rural/urban divide

Nobody wants to be in a state with a lot of COVID-19 cases. Nobody except perhaps an epidemiologist trying to study how the disease spreads.

In a paper in today’s Proceedings of the National Academy of Sciences, a team from Emory University (in Atlanta) and the Georgia Department of Public Health, took advantage of the fact that their state ranks 6th in the U.S. in per capita cases to hone in on just how the disease spreads.

They looked at data from the five counties in the state with the most cases, looking for, among other things, superspreader events.

Four of these counties were in the greater Atlanta area; the fifth was rural.

In addition to tabulating cases, they used detailed maps of population density, plus data obtained from Facebook on how how much people traveled during the early months of the pandemic, based on cell phone data. (Such information has been used in other types of studies based on aggregates large enough protect individual privacy.)

They found two really interesting things.

One was that before Georgia asked people to shelter in place, the “reproductive rate” (sometimes called R0 or R-naught) of the virus was 3.3 overall…and 5.2 in rural areas.

In less technical terms, the R0 reflects the number of other people each infected person, in turn, infects. These figures mean that, prior to the shelter-in-place order, each person who became infected with the virus in the five counties infected an average of 3.3 other people.

That’s a lot, because it means that 100 cases quickly becomes 330, then 1,090, and then 3,600, etc.

But an R0 of 5.2 is vastly worse. It means that 100 cases just as quickly become 520, 2,700, and 14,000.

So something different is going on in rural areas, which is interesting because intuitively we’d think that rural areas, with their greater “normal” levels of social distancing would be less prone to spreading the virus.

The next finding was good news: Once people adapted to shelter-in-place (which took about 1-2 weeks), the R0 quickly began to drop, falling below 1.0 in about 3 weeks.

This is important, because an R0 of less than 1.0 is the level at which the number of new cases begins to drop, rather than rise. It’s also not terribly surprising. We’ve seen the same in other states, where, a few weeks after people get serious about fighting the disease, the level of new cases does begin to decline (however slowly).

But there was a third, very interesting, finding.

One of the major causes of COVID-19 transmission, we’ve known for several months are super-spreader events…and super-spreader individuals.

The former are gatherings such as indoor church services, funerals, parties, etc., in which large numbers of people are simultaneously infected, presumably because an infected person attends and manages to have close contact with a lot of others, all in a single day.

Super-spreader individuals are people who individually infect a large number of people—not just 3.3 or 5.2, but 20, 50, or 100. The record may go to a woman known as Patient 31 in South Korea who, knowing she was sick, went to a megachurch and single-handedly set off a cluster of 5,080 cases.

That was previously well known. What the new study adds to the mix is the discovery that as the pandemic evolved, at least in Georgia, super-spreaders became a bigger factor in producing new cases—and that they were more likely to do so in rural areas.

What does this mean for us as we approach our sixth month of COVID-19 fears and frustrations?

Three things, as I see it.

(1) We need to be careful not to let our guard down, however tired we may be of the present precautions. These precautions work. This study proved that it’s possible to beat the virus down, with patience.

(2) Rural areas aren’t immune. In fact, they may be more vulnerable than  cities, perhaps because their normal social distancing makes them feel safer. Which, most likely, they are, most of the time. Until people gather for large events and undo all of their natural precautions in a single day. (This warning also applies to urban folks. Just because you’re normally in a fairly small bubble doesn’t mean you are protected if you occasionally venture out of it into a crowded bar, etc.)

(3) We need to really think carefully about re-opening schools. If you’re looking for a good setting for a super-spreader event, sadly, schools look like depressingly good candidates.

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