"We are dealing with a different virus now."
That's what Dr. Anthony Fauci, White House chief medical advisor, told NPR Tuesday about the Covid delta variant.
"This is not the original virus that we were dealing with. This has different capabilities much more efficient in transmitting from person to person," he said, explaining why the Centers for Disease Control and Prevention's changed its mask guidelines.
Fauci said the CDC reversed its mask guidance on Tuesday — recommending that fully vaccinated people go back to wearing masks indoors in places where there are high transmission rates — because delta is now the dominant strain in the U.S. In May, when the CDC announced that fully vaccinated people could ditch their masks, the more transmissible delta variant only represented about 1-2% of infections in the U.S.
Delta is highly transmissible and acts "uniquely differently from past strains of the virus," CDC director Rochelle Walensky said during a call with reporters Tuesday.
Research suggests that delta is as much as 60% more transmissible than the previously dominant alpha variant.
And "we know now as a fact that [vaccinated people with Covid] are capable of transmitting the infection to someone else," Fauci told NPR.
Breakthrough infections, although rare, occur when a vaccinated person is infected with the virus. It had been unclear whether a vaccinated person would have enough virus in their system to pass it to others. But new data suggests that, yes, fully vaccinated people who are infected with the delta variant can transmit the virus to other people, Walensky said.
So the CDC recommends that even people who are fully vaccinated mask up indoors if they live in places where there is "substantial transmission" (counties that have 50 to 100 cases per 100,000 residents over a seven-day period) or "high transmission" (more than 100 cases per 100,000 people over seven days). Choose your state and county on this CDC map to determine whether you need to wear a mask in your area.