ANALYSIS: Why Vaccinated People Still Need to Wear a Mask

The new Covid-19 vaccines from Pfizer and Moderna seem to be remarkably good at preventing serious illness. But it’s unclear how well they will curb the spread of the coronavirus.

That’s because the Pfizer and Moderna trials tracked only how many vaccinated people became sick with Covid-19. That leaves open the possibility that some vaccinated people get infected without developing symptoms, and could then silently transmit the virus — especially if they come in close contact with others or stop wearing masks.

If vaccinated people are silent spreaders of the virus, they may keep it circulating in their communities, putting unvaccinated people at risk.

“A lot of people are thinking that once they get vaccinated, they’re not going to have to wear masks anymore,” said Michal Tal, an immunologist at Stanford University. “It’s really going to be critical for them to know if they have to keep wearing masks, because they could still be contagious.”

In most respiratory infections, including the new coronavirus, the nose is the main port of entry. The virus rapidly multiplies there, jolting the immune system to produce a type of antibodies that are specific to mucosa, the moist tissue lining the nose, mouth, lungs and stomach. If the same person is exposed to the virus a second time, those antibodies, as well as immune cells that remember the virus, rapidly shut down the virus in the nose before it gets a chance to take hold elsewhere in the body.

The coronavirus vaccines, in contrast, are injected deep into the muscles and stimulate the immune system to produce antibodies. This appears to be enough protection to keep the vaccinated person from getting ill.

Some of those antibodies will circulate in the blood to the nasal mucosa and stand guard there, but it’s not clear how much of the antibody pool can be mobilized, or how quickly. If the answer is not much, then viruses could bloom in the nose — and be sneezed or breathed out to infect others.

“It’s a race: It depends whether the virus can replicate faster, or the immune system can control it faster,” said Marion Pepper, an immunologist at the University of Washington in Seattle. “It’s a really important question.” ...