What to Know About Breakthrough Infections and the Delta Variant

Scientific understanding of the coronavirus variant is changing quickly. Here’s a recap of the most important findings.

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.