A Negative Test Doesn’t Mean Biden Is in the Clear

But other factors surrounding the debate suggest the likelihood of infection is low

Joe Biden and Donald Trump stood at podiums set 8 feet apart during the debate. Photo: Olivier Douliery-Pool/Getty Images

Could Democratic nominee Joe Biden have been infected with the novel coronavirus during the debate on Tuesday? It’s unlikely, and the fact that he tested negative today is a good sign, but it doesn’t mean he’s entirely in the clear.

It can take anywhere from two to 14 days for Covid-19 symptoms to emerge. During this incubation period, the virus is replicating until its levels get high enough for it to be detected with a test, and for the infected person to be contagious. A few days after that, the immune system really kicks in and symptoms start to develop. In other words, a negative test and no sign of symptoms three days after a potential exposure doesn’t mean they won’t emerge later.

“If somebody tests negative, they may be incubating the virus,” says infectious disease expert and epidemiologist Celine Gounder, MD. “Just because you have a negative test doesn’t mean you don’t have it.”

However, Gounder says, given that President Trump only tested positive for the virus on Thursday — but not on Tuesday or Wednesday — he likely was not shedding enough of the virus earlier in the week to be contagious at the debate. “If somebody is negative on their test, that to me says they’re not infectious to other people and are not passing it on to other people,” she says.

What’s more, aerosol scientist Jose Jimenez, PhD, says that the debate was well set up to reduce the chances of transmission between Biden and the president.

Scientists now agree that the virus is spread through larger droplets and smaller aerosols that get expelled when people are breathing or speaking. The larger particles typically fall to the ground within six feet, but the smaller aerosols can remain afloat for longer and be carried on gusts of air. Trump and Biden were relatively far apart on the debate stage, which makes the risk of droplet transmission low. Jimenez says that the size of the venue and its ventilation system helped mitigate the risk of airborne spread as well.

“These places have to comply with regulations about how much air has to be coming in at all times, and these…