This article has been updated to reflect news developments.
President Trump and the first lady have tested positive for the coronavirus. Now everyone who has been anywhere near them will need to be tested too, ideally multiple times.
That includes Joe Biden. He tested negative today, after his debate with Mr. Trump on Tuesday. But we need to be careful about how much faith we put in negative results, because they don’t always mean what we hope they do. A negative test tells us only one thing: That in the moment someone was tested, the swab didn’t pick up any viral particles. It doesn’t tell us the virus isn’t there at all, or that it won’t be there in a day or two, even in a few hours.
“There’s a mind-set out there that the test gives you a definitive answer: If it’s positive, you have the disease. If it’s negative, you don’t,” said Colin P. West, a physician and biostatistician at the Mayo Clinic in Rochester, Minn. “That’s just not true.”
Sometimes, there is little reason to doubt a negative result. But if a person believes a negative result that turns out to be wrong, the results can be disastrous. “You’re falsely reassured,” said Steven Woloshin, a professor at the Dartmouth Institute for Health Policy and Clinical Practice. “You think you don’t carry the virus, and then you may go around and spread it to other people, which can have really serious consequences.”
We don’t know if the president was already incubating the virus when he tested negative in advance of the debate, but he could have been. The same could be true of Mr. Biden now, if he goes out on the campaign trail.
Negative tests are tricky to interpret in part because of the issue of timing. After people are exposed to the coronavirus, they incubate it for several days before testing positive or showing symptoms. In a systematic review of seven studies published in May, researchers at Johns Hopkins University analyzed how long this process typically takes. They found that the most widely used test, called a P.C.R. test, was most likely to come back positive eight days after an exposure to the virus, which tended to coincide with a person’s third day of symptoms. Still, even on this ideal day, the tests came back negative on average in 20 percent of people who later tested positive.
We don’t know when President Trump was first infected. Late in the day on Thursday, the administration announced that one of his senior aides, Hope Hicks, had tested positive. Around 1 a.m. Friday, the president tweeted that he and the first lady had as well. It’s certainly possible that he had the coronavirus at the debate on Tuesday and could have infected Mr. Biden, even if he didn’t have symptoms.
Based on the research, we may not know for another week. Even though Mr. Biden’s initial tests came back negative, it doesn’t mean he’s in the clear.
After a possible exposure, experts advise waiting at least three or four days before getting a test — ideally self-quarantining during the wait — and then, if it’s negative, getting another test a few days later. There’s virtually no chance a test will work in the first day or two after exposure, when “the false negative rate is really high — somewhere between 60 and 100 percent,” said Lauren Kucirka, one of the authors of the Johns Hopkins study.
In another study published in June, researchers working with data from NewYork-Presbyterian laboratories found that 18.6 percent of people who initially got a negative P.C.R. test between March and May and were then retested got a positive result within several days.
Even if a person waits long enough to get a test, it can still come back negative when that person actually has the coronavirus. Some infected people have been found to harbor the virus deep in their lungs instead of in their upper respiratory tract. Samples can also be collected, stored and handled improperly, causing tests to fail.
To gauge how trustworthy a negative test result is, doctors recommend asking other risk-related questions. These questions are based on Bayes’s theorem, an approach rooted in probability theory. They include: Does the person have Covid-19 symptoms? Is the virus circulating widely in the community? Has the person been exposed to someone with the coronavirus, or attended large gatherings? And so on. The more “yes” answers, the more people should doubt a negative test result.
Mr. Biden was in the same room as Mr. Trump for 90 minutes, even though they stood far apart. If the president was contagious, and some viral particles traveled Mr. Biden’s way, it’s possible he could have been infected.
People in the White House who have spent more time with Mr. Trump and Ms. Hicks should be even more concerned. Those who have good reason to think they’ve been exposed should essentially consider a negative test meaningless. They need to quarantine for two weeks no matter what.
Given this uncertainty, we are going to need to be patient. The negative tests we might want to celebrate over the next few days won’t actually tell us much. We can only hope for the best.
Melinda Wenner Moyer (@lindy2350) is a science and health writer and the author of a forthcoming book on raising children.
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