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Updated at 2:20 p.m. ET on October 4, 2020.

“I don’t wear masks like him,” President Donald Trump said during Tuesday night’s presidential debate, deriding his opponent, former Vice President Joe Biden. “Every time you see him, he’s got a mask.” But at nearly 1 a.m. eastern time today, Trump announced that he had tested positive for the SARS-CoV-2 coronavirus, which causes COVID-19.  

As president, Trump’s dire mishandling of the U.S. pandemic response has contributed to the deaths of about 200,000 Americans and at least 7.2 million infections. But now that he himself is sick, one has to wonder: How many people has Trump personally infected? And could Biden be one of them? Encouragingly, Biden tested negative for the virus today, but “a negative test doesn’t say he’s completely in the clear,” says Angela Rasmussen, a virologist at Columbia University. Several factors suggest that he could have been exposed to the virus during the debate, and should continue to take precautions for at least another week.

First, there are the conditions of the debate itself. The coronavirus mostly spreads through the air, traveling from the nose and mouth of an infected person in either large, wet particles (droplets) or smaller, drier ones (aerosols). Most droplets fall to the ground within six feet of their source, and Trump and Biden were clearly standing farther apart than that. But “aerosols behave like cigarette smoke and don’t stop at six feet,” says Linsey Marr, who studies airborne-disease transmission at Virginia Tech. “Imagine Trump was smoking the whole time. Would Biden have been exposed to some of that?”

When thinking about COVID-19 transmission, there are no absolutes, only probabilities. The distance between Trump and Biden lowered the odds of infection: The farther Trump’s aerosols traveled, the more dilute they would have become. But almost everything else about the debate increased the risk that those aerosols could have found their way into Biden’s nose. People release about 10 times more aerosols when talking than when breathing silently, Marr says, and even more when talking loudly. Trump certainly did that—for 90 minutes, in an enclosed space, without wearing a mask, and often in Biden’s direction. “It wasn’t a one-off cough by someone in the audience,” says Joseph Allen, an environmental-health expert at Harvard University. “It was one and a half hours of constant emissions.” These are the same conditions that make bars and restaurants such risky venues for COVID-19 transmission.

The debate took place inside a spacious atrium at Case Western Reserve University, in Cleveland, and ventilation—how often air is circulated, how thoroughly it is filtered, and where the vents are—would have affected Biden’s risk of catching viruses from Trump. These factors are unclear. Under advice from the Cleveland Clinic, the university detailed several steps to protect the health of participants, including limited attendance, disinfectant measures, and extra space between seats. “But what was notably absent was any mention of healthy building strategies, like filtration and ventilation,” Allen says. “These are things that should be thought about all the time, and certainly in this case.”

Second, there’s the matter of Trump himself. One of the most crucial questions—and the biggest unknowns—is whether he was contagious during the debate. The incubation period for COVID-19—the time between becoming infected and developing symptoms—can last two to 14 days, but is most often four to five days. But people can be infectious one to three days before their symptoms first emerge. Assuming that Trump first developed symptoms on Wednesday (when aides say he was feeling poorly) or yesterday (when he got the test that led to his diagnosis), he would most likely have been infected last weekend, and been infectious on the night of the debate.  

In a statement, the Cleveland Clinic said that “everyone permitted inside the debate hall tested negative for COVID-19 prior to entry.” After the debate, moderator Chris Wallace said that both campaigns were on the “honor system,” responsible for their own testing prior to arriving in Cleveland. But no test is perfect, and the fastest ones are more likely to produce false-negative results, failing to detect viruses that are actually present. Even the more sensitive tests have false-negative rates of 38 percent on the first day of symptoms, and almost 100 percent four days before that. In those early days, “you’d expect virus levels to be increasing exponentially,” Rasmussen says. “It’s completely plausible that you could test negative, and a few hours later, test positive.”

The variability around both the tests and the disease itself makes it hard to reconstruct a firm timeline of what happened to Trump and those close to him. For example, Hope Hicks, a senior adviser, tested positive for COVID-19 on Wednesday, a day before Trump did. But this timing doesn’t mean that Hicks gave the virus to him, or even that she was infected before he was. Trump could have been infected first but slower to develop symptoms. He might have had several false-negative test results, while Hicks was first to have a true positive. Hicks might have been tested more frequently than Trump: Anyone in proximity to the president is tested daily, and although the White House says the president is tested “multiple times a day,” Trump himself has said he is tested only once every two or three days. “This visceral response that he got it from Hicks—we can’t say that,” says Saskia Popescu, an infection preventionist at the University of Arizona. “We can only loosely understand the general timeframes.”

Both Hicks and Trump could have caught the virus from a third person, or from completely different people. Indeed, there are plenty of candidates. Many of Trump’s supporters and aides have been vocal about not wearing masks, and frequently came into close contact with other people in indoor spaces. Melania Trump has also tested positive, as have Ronna McDaniel, the chair of the Republican National Committee, Senator Mike Lee of Utah, and the University of Notre Dame president, John Jenkins. Many of them were at the Rose Garden event on September 26, when Trump officially nominated Amy Coney Barrett to the Supreme Court. Others in the White House, at Trump’s rallies, and at Tuesday’s debate could have been exposed. Contact tracing in these situations will be extremely difficult.

“Everyone who was at the debate should now be quarantining as much as possible, monitoring themselves closely for symptoms, wearing masks, and keeping their physical distance as much as possible,” says Eleanor Murray, an epidemiologist at Boston University. That goes for Biden, too, despite today’s negative test. After being exposed to the virus, half of the people who go on to show symptoms are symptomatic by day five—that would be Sunday for Biden, if he was exposed during the debate. About 98 percent of people are symptomatic by day 12, which would be next Sunday. If Biden is still testing negative a week from now, “it’ll be a good sign that there’s little likelihood of having been infected,” Murray says. Until then, he has to wait.

The image of Trump shouting at Biden on a national stage raises the specter of the former infecting the latter. But as ever, the pandemic says as much about the world we live in as the behavior of individuals. That we are even weighing the possibility of the incumbent president inadvertently infecting his opponent with a pandemic virus during a nationally televised event should be an indictment of America’s laxity in dealing with the pandemic—its reticence to restrict indoor activities, which give the virus the best chance of finding new hosts; its failure to enforce measures like masks, which might render such events safer; and its continuing inability to control the disease, which many nations have brought to heel.

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