Disease detectives at Boston’s Broad Institute used genomic sequence data to trace the outbreak to just a handful of infected people, including one who was the likely source of more than 80 percent of cases.

“However, if you are vaccinated and you get infected, you’re less likely to spread the virus than if you’re unvaccinated.”.

He and colleagues studied the immune responses of 35 vaccinated people tested for COVID-19 in the Provincetown outbreak.

Those who got a breakthrough infection had a dramatic increase in levels of virus-blocking antibodies and virus-killing T cells, including 34-fold higher neutralizing antibody levels against the delta variant, compared with those who weren’t infected.

“We think that this is likely the reason why vaccinated people who get breakthrough infections generally have a mild course of disease in the vast majority of cases, because they have a rapid onset of very potent antibody and T cell responses that likely control the virus,” Barouch said over Zoom.

The pattern is likely to play out on a larger scale as the coronavirus continues to circulate worldwide, including among people with varying levels of immunity.

Less than 10 percent of people in much of sub-Sahan Africa have been immunized, data from the Bloomberg COVID-19 Vaccine Tracker show

“We would, of course, prefer that people have a bump in their immune responses from getting additional booster vaccines,” said Barouch, who helped develop the Johnson & Johnson COVID-19 shot

“But in reality, there will be cases of breakthrough infections and those are not rare and they’re not overly concerning because, in the vast majority of cases, they result in only mild disease.”

More studies on breakthrough infections are needed, including to gauge the risk of persistent symptoms often called long COVID, and the levels of immunity that protect against it, he said