“Because of this data … I’m very worried about variants for which vaccines may be less effective,” Chiu told The Chronicle. “We have in the San Francisco Bay Area a highly vaccinated community. In that setting, if we do see another outbreak or peak of cases, it’s going to be due to a variant that’s even more resistant than delta.”

The study was posted on a pre-print server Wednesday and has not been peer-reviewed. It seeks to better understand the link between vaccination and the evolution of variants.

During the time of the study between Feb. 1 and June 30, San Francisco went from 2% vaccinated to 70% vaccinated. Researchers sequenced samples from 1,373 people in San Francisco with COVID-19. Of those cases, 9%, or 125 people, had breakthrough infections. Fully vaccinated people were more likely than unvaccinated people to be infected with variants carrying mutations associated with decreased antibody neutralization.

“As the population gets more vaccinated, and this makes sense intuitively, it’s more likely variants that’ll be selected for are those that are more resistant,” Chiu said.

However, because variants can come with trade-offs — a variant that’s more infectious may not be more antibody-resistant, and vice versa — it’s possible that a future predominant variant that’s more antibody-resistant may not be more infectious.

The study also found that people with breakthrough symptomatic infections are probably as infectious as unvaccinated individuals with COVID. This echoes the findings from a Cape Cod, Mass., study released by the CDC last month that examined an outbreak that was mostly delta cases. Chiu’s study, though, expanded on that, finding that is true for several circulating variants, not just delta. The Bay Area study was done when alpha, delta, gamma, epsilon and other variants were circulating in the community.

Moreover, the study found that people with breakthrough symptomatic infections have much higher viral loads — 400 times higher, and thus likely much more infectious — compared to people with breakthrough asymptomatic infections. This is much more pronounced than in unvaccinated people, where the difference between the viral loads of symptomatic and asymptomatic people is tenfold and not significant, Chiu said. Asymptomatic breakthrough cases are probably not a major source of spread, he said.

“What that implies is symptomatic vaccinated breakthrough infections may be a potential source of spread,” he said. “It highlights the importance of even if you’re vaccinated, if you have symptoms, it’s important you get diagnosed, and that you wear a mask and stay at home to avoid spreading it to others. Because you’re potentially as infectious as someone who’s unvaccinated.”