Biden has tested negative. That doesn’t mean he’s in the clear.
Trump Reportedly Arrived to the Debate Too Late to Be Tested by Organizers
https://slate.com/news-and-politics/2020/10/trump-no-coronavirus-test-debate-wallace.html
President Donald Trump was not tested for the coronavirus by the Cleveland Clinic prior to the first presidential debate on Tuesday night, according to moderator and Fox News anchor Chris Wallace.
The president and first lady’s positive coronavirus tests, announced early Friday, shocked the country and have prompted speculation as to whether Joe Biden may have contracted the virus from Trump at the debate.
https://slate.com/technology/2020/10/biden-trump-debate-transmission.html
More than half of the guests on Trump’s side at the debate reportedly declined to wear masks, while everyone on Biden’s side was wearing masks. A doctor with the clinic even approached some of Trump’s guest and offered them masks, but they refused.