Why should vaccinated people wear masks? UCSF expert gives his take

Counties across California are recommending that people wear face masks, even those who are fully vaccinated, in public indoor places as the highly contagious delta variant drives an uptick in COVID-19 cases.

Solano County is the only county in the Bay Area that hasn't issued a mask advisory, and several counties outside the region are calling for a return to masking indoors, including Monterey, Santa Cruz and San Benito. Los Angeles County is requiring masks in indoor public spaces.

If you're fully vaccinated, you may be wondering why you should consider wearing a mask indoors. Research shows the vaccines are highly effective and even if you do get a breakthrough case — that is, become infected even though you're vaccinated — you're unlikely to experience severe symptoms and highly unlikely to land in the hospital.

SFGATE checked in with one of the nation's leading COVID experts, Dr. Bob Wachter, a professor and chair of the department of medicine at UCSF, to give his take on why he's following the recommendation.

"I'm still comfortable going maskless in a small social gathering when I'm 100% sure everyone is vaccinated or 100% sure that any unvaccinated person is wearing a mask," Wachter wrote in an email. "I don't know how I can be sure about the latter, and even the former is tricky unless it's a small, trusted group."

Wachter gave seven reasons in his email for why he thinks it may be advantageous to mask at gatherings where there are likely to be unvaccinated people who may or may not be wearing masks:

1) "The rate of breakthrough infections is small but very real."

2) "The rate of breakthrough infections with delta is likely somewhat higher than with the original virus; this number is still being debated."

3) "Delta might be more likely to lead to a serious infection than the original virus; at age 63, I'm already in an moderate risk group for hospitalization and death."

4) "Though the chances of long COVID with a breakthrough infection seem to be low, there is no rigorous evidence on this and we certainly have seen mild primary infections cause prolonged symptoms."

5) "I'd guess that the chances of catching and transmitting COVID after vaccination are low (as they were with the original virus), but this too is somewhat uncertain."

6) "Given #5, I see going maskless indoors as not only potentially putting myself at risk but also others — particularly immunosuppressed people, children, and those who have chosen not to be vaccinated."

7) "While the vaccines seem to be holding up quite well, I'm now more than 6 months out from my vaccination and we don't know duration of effect, particularly with delta."