The Delta Variant: What Scientists Know

The spread of the super-contagious Delta variant has prompted new restrictions around the world and spurred stark new warnings from public health officials.

“The Delta variant is more aggressive and much more transmissible than previously circulating strains,” Dr. Rochelle P. Walensky, director of the Centers for Disease Control and Prevention, said at a White House briefing last week. “It is one of the most infectious respiratory viruses we know of and that I have seen in my 20-year career.”

An internal C.D.C. document, a copy of which was obtained by The New York Times, said that Delta was as transmissible as chickenpox. Although the vaccines continue to provide strong protection against severe disease and death, fully vaccinated people can be infected by, and transmit, Delta, the agency noted. Given these facts, it was time to “acknowledge the war has changed,” the document said.

This week, these observations prompted the C.D.C. to change the recommendations it released in May, which had said that fully vaccinated people did not need to wear masks indoors. The agency now says that in virus hot spots, everyone, regardless of vaccination status, should wear masks in indoor public spaces. It also recommended universal masking in schools this fall.

The World Health Organization, citing the rise of Delta, the dearth of vaccines and high rates of community transmission in many parts of the world, has also encouraged fully vaccinated people to continue wearing masks.

First identified in India, Delta is one of several “variants of concern,” as designated by the C.D.C. and the World Health Organization. It has spread rapidly across the world and poses a particular threat in places where vaccination rates remain low.