Former Vice President Joe Biden and others around President Trump have tested negative for COVID-19 – but that doesn’t mean they’re totally in the clear, experts said Friday.

A negative test only means someone isn’t shedding much virus at that moment. It doesn’t say anything about what may be brewing in the person’s body. 

Trump, who was known to get tested daily for COVID-19, presumably tested negative on Tuesday, the day of his debate with Biden, which reduces the chance the President passed the virus on to his political opponent.

But even if he tested negative in the morning, it’s still possible he was contagious by the time of the evening debate, said Dr. Michael Mina, an infectious disease epidemiologist at the Harvard T. H. Chan School of Public Health.

The virus comes on quickly, and is most infectious during the day or two before symptoms appear, and a day or two after someone feels sick, if they ever do.

So, Biden is “not completely out of the woods yet,” Mina said.

He would probably not have a positive test if he had been exposed on Tuesday because most people don’t test positive in the first day or two after exposure, said Shruti Mehta, an infectious disease epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

The same would have been true of Trump on Tuesday, if he had only been exposed over the weekend. “Trump could have been incubating virus and infectious before he tested positive and/or had symptoms,” she said via email.

Both men did not wear masks during the 90-minute televised event, often shouting or speaking in loud voices, which is known to spread more viral particles. And if Trump had been contagious and Biden walked directly behind him onto the stage, Biden could have passed through a cloud of particles, potentially inhaling some, Mina said.

Although most people who catch COVID-19 have relatively mild symptoms if any at all, both men are at higher risk for serious illness because of their age, gender and potentially other factors. Trump, who is 74, runs a five-times higher risk of hospitalization than a 20-something, and Biden’s risk is eight times higher, according to the U.S. Centers for Disease Control and Prevention, because he is nearly 78.

Only time will tell if Trump passed the virus on to others. It’s still unclear who gave it to him or when he might have become infectious. 

Since Trump began reporting symptoms on Thursday “he could have been shedding virus at a high rate on Tuesday evening,” said Dr. Monica Gandhi, a professor of medicine at the University of California San Francisco.

“The median incubation period is 5 days so he should test again on the weekend,” she said via email.

The CDC recommends that anyone with a known exposure to the virus quarantine at home for 14 days, separate themselves from others, and monitor their health. But since Biden and top administrative officials have access to regular testing, they may be able to come out of quarantine sooner.

“In terms of quarantining, the general public does not have access to daily tests but if VP Biden tests this weekend and early next week negative, I would be confident he did not contract the infection,” Gandhi said.

Contact Karen Weintraub at [email protected]

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