“There’s risks on either side,” Anthony S. Fauci told The Washington Post, warning that shifting to a single-dose strategy for the Pfizer-BioNTech and Moderna vaccines could leave people less protected, enable variants to spread and possibly boost skepticism among Americans already hesitant to get the shots.

“We’re telling people [two shots] is what you should do … and then we say, ‘Oops, we changed our mind’?” Fauci said. “I think that would be a messaging challenge, to say the least.”

Fauci said he spoke on Monday with health officials in the United Kingdom, who have opted to delay second doses to maximize giving more people shots more quickly. He said that although he understands the strategy, it wouldn’t make sense in America. “We both agreed that both of our approaches were quite reasonable,” Fauci said.