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President Trump defied public health guidance this election season with his big rallies. There's lots of concern those events would be the source of coronavirus outbreaks or superspreaders. So what does the evidence say? Will Stone reports.
WILL STONE, BYLINE: In late September, not long before getting COVID-19 himself, President Trump descended the stairs of Air Force One for one of his signature campaign rallies.
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PRESIDENT DONALD TRUMP: Thank you very much. Hello, Newport News. I know it well.
STONE: Staring back at the president - lots of red hats, people shoulder to shoulder and very few masks. Just a few miles away, Zach Nayer, a medical resident, was horrified.
ZACH NAYER: On the inside of the hospital - where I am treating COVID patients, you know, seeing a lot of really sick, sick people die alone - it's just so disheartening.
STONE: You don't need a medical degree to see that Trump's campaign rallies are primed for spreading the coronavirus. They violate the CDC's own guidance to wear masks and socially distance. But Nayer wanted to know if the president's rallies actually seeded outbreaks, and so did his friend Usnish Majumdar, a medical student in New York City.
USNISH MAJUMDAR: We sort of, like, fleshed out all the different ways we might approach this using our statistics backgrounds. And then we were like, OK, but, like, what would be the minimum required to just suggest that there may have been an effect?
STONE: They collected data on how many coronavirus cases were recorded in 14 rally locations in the two weeks before and after.
NAYER: You know, basically, half of the places where he's been to had pretty noticeable spikes.
STONE: On their graph. You can see some places had big jumps, like Bemidji, Minn., and Old Forge, Pa. In others, it was less pronounced. They say there's an undeniable association, but the study had major limits.
MAJUMDAR: The finding that compelled us to say, like, look, there might be an effect, was simply that there were, in aggregate, more cases after these rallies than there were before.
STONE: They published their data on the website Stat, along with an op-ed, not in a peer-reviewed scientific journal. And it turns out getting at this question is actually pretty hard.
JOSH SHIFFER: It was a nice effort to do that with the best available data that they had.
STONE: That's Dr. Josh Shiffer, an infectious disease researcher at the Fred Hutchinson Cancer Research Center.
SHIFFER: I think it's really weak evidence, but I want to be careful saying that.
STONE: Because it's definitely plausible. And Shiffer says, unfortunately, the U.S. is woefully behind on contact tracing, which could identify where people got infected.
SHIFFER: That's how you could make a truly compelling case.
STONE: One approach would be to compare people who got COVID in the two weeks after the rally with healthy people in the same area who didn't get COVID. That's how Ben Lopman, a professor of epidemiology at Emory University, might go about it.
BEN LOPMAN: And we would look at their risk. What were they exposed to? You know, were cases more likely, for example, to have attended a mass gathering like a rally?
STONE: Lopman says it's tempting to look at cases over time and draw conclusions, but there are lots of variables. What else was going on that could have spiked cases?
LOPMAN: You never know exactly where people got infected. So it's hard. It's hard to do these studies.
STONE: In their op-ed, Majumdar and Nayer say the huge Black Lives Matter protests did not appear to spike cases like these rallies. It's a contentious claim. Dr. David Hamer at Boston University says he worried those might cause a surge, but...
DAVID HAMER: In most parts of the country where there were fairly major protests, these were not seen. I mean, there were maybe a few small increases but nothing really substantial.
STONE: All of this is pretty crude evidence for a skeptical epidemiologist, he says, but it's what we have to work with.
For NPR News, I'm Will Stone.
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