From FiveThirtyEight:
There’s a hypothetical mechanism that could allow this to happen biologically, said Deepta Bhattacharya, a professor of immunobiology at the University of Arizona. And that mechanism is … well … it’s boogers and phlegm.“So, the virus enters in through the upper respiratory tracts, either through your Nose or your throat. And those are protected by a Mucous Layer. And so that mucous layer is good at slowing things down from getting into you. But it also acts as a barrier for things like antibodies, and certainly for cells from getting out and meeting the virus as it comes in,” he said.
[Your immune cells] might not be able to neutralize the ones resting in your nose, on the other side of your mucous barriers. Those COVID-19 viruses wouldn’t hurt you, but they still might be able to replicate and shed — coughed back out of your nose and mouth....
