CDC: 40 Million Americans May Already Be Infected. So Is 'Herd Immunity' Around The Corner?
'CDC signals that 40M Americans may already be infected, so is herd immunity around the corner?' By Mark Sumner, Daily Kos Staff, July 21, 2020.
Right now, 1.5% of all Americans have been confirmed as having COVID-19. That’s an amazing number—4 million confirmed cases—far more than any other nation. In fact, despite having just 4% of the globe’s population, the United States still has about a quarter of all cases of COVID-19—a remarkable measure of the Trump White House’s failure to provide anything that resembled leadership in the face of this enormous crisis.
> However, on Wednesday the Centers for Disease Control and Prevention (CDC) published analysis that matched the results of another recent study, indicating the true number of cases in the United States is likely around 10 times the number of confirmed cases. That would mean that as much as 15% of the population may have been exposed at this point, with some areas having a rate of exposure as high as 35%.
In some ways this is terrifying—the number of active cases of COVID-19 in the United States right this moment is likely around 20 million. That’s the kind of number that is daunting to the very idea of case tracing and isolation. If the full 4,300 person staff of the Census were to work on nothing else, they would each be dealing with 4,650 cases—and that’s if everyone just stood still long enough to let them work it through. But is there also good news in this? Doesn’t it bring the United States closer to “herd immunity?” And what’s with the widely circulated notion that COVID-19 immunity can be achieved with a relatively small number of infections?
There’s not a magic number that represents herd immunity. Actually, herd immunity is not even a thing—nothing makes the remaining population one bit more immune to a disease just because some other percentage already has it. 99.999% of the population might have been past COVID-19, but were you seated across from someone with an active case for some period of time, and you were part of that 0.001% who had not been infected … odds are you’d be on your way to a case of COVID-19.
All that herd immunity really amounts to is a decrease in the rate of transmission. Since the start of the pandemic, the rate of transmission for COVID-19 (often expressed as R0) has been somewhere between 2.4 and 3.0. That’s the average number of people infected by the average person with an average case of COVID-19 (on, you know, average). But that number is far from constant and far from universal. It’s already been demonstrated that some people are behind dozens of cases. Others infect no one. That can vary because of population density, housing, infrastructure, and social distancing rules, just for a start.
If you had a densely populated city where everyone tended to travel by public transport, lived in multi-family dwellings, and local authorities were doing nothing to interfere with the spread of the disease, the R0 of COVID-19 could easily reach the double-digit values usually assigned to diseases like measles or smallpox. On the other hand, a rural area where everyone travels in their own cars, lives in separate homes, and is lucky enough to have authorities enforcing a mask mandate might well have an R0 below 1—the point at which the outbreak can no longer sustain itself and gradually fades away...
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