What to Know About Breakthrough Infections and the Delta Variant

Citing new evidence that vaccinated Americans with so-called breakthrough infections can carry as much coronavirus as unvaccinated people do, the Centers for Disease Control and Prevention last month urged residents of high-transmission areas to wear masks in public indoor spaces, regardless of their vaccination status.

The announcement reversed the agency’s recommendation in May that vaccinated people could forgo masks. The vaccines remain highly effective at preventing severe illness and death, but the highly contagious Delta variant and persistent vaccine refusal have taken the country in an unexpected direction. Infections have spiked to the highest levels in six months.

“If the war hadn’t changed, I wouldn’t have felt the need to take such a widely unpopular action,” Dr. Rochelle Walensky, the C.D.C.’s director, said in an email.

Dr. Walensky has repeatedly said that breakthrough infections are extremely rare. But the agency does not tally national figures on breakthrough infections that don’t result in hospitalization or death, and, in any event, its numbers lag by a few weeks. The exact incidence of these infections, as well as their outcomes, is unknown.

Breakthrough infections seemed to be vanishingly rare when previous versions of the coronavirus dominated in the United States. But recent outbreaks suggest that the numbers may be higher with the arrival of the Delta variant.

“A modest percentage of people who are fully vaccinated will still get Covid-19 if they are exposed to the virus that causes it,” Dr. Walensky said in the email.