“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.