Breakthrough infections are expected because no vaccine is 100 percent effective. In rare cases, fully vaccinated people can get seriously ill and die from Covid-19, but the vast majority of breakthrough cases have been mild or asymptomatic. That's because the vaccines act like screens to block most — but not necessarily all — virus particles from invading the body.
Different factors influence the strength of the screen and how many tiny virus particles are able to make it through the barrier, said Dr. Sarah Fortune, an immunologist at Harvard's T.H. Chan School of Public Health.
"These variants are more transmissible, so they're better at getting through the screens," she said. "The other factor is how much virus is out there trying to get in, and that's determined by vaccination rates in your local community. It's how much virus you're being exposed to."
Vaccines can also lower the amount of virus in the body, which may limit the ability of people with breakthrough infections to spread it to others, although the effect is not yet well understood. More research is needed to gauge what effect asymptomatic breakthrough cases, in particular, have on transmission.
"It may be that for the vast majority of vaccinated people who get infected, they just don't make enough virus to infect another person," said Dr. Janko Nikolich-Žugich, an immunologist and professor of medicine at the University of Arizona.
Darnell, of Rockefeller University, said the recent rise in breakthrough infections hasn't been associated with a similar increase in hospitalizations or deaths, which is encouraging evidence that vaccines seem to be holding up well, in spite of new and emerging variants.
Nothing to see here. Move along.