The United States must stick to a two-dose strategy for the Pfizer-BioNTech and Moderna COVID-19 vaccines, top US infectious disease official Anthony Fauci told the Washington Post newspaper.

Fauci said that delaying a second dose to inoculate more Americans creates risks.

He warned that shifting to a single-dose strategy for the vaccines could leave people less protected, enable variants to spread and possibly boost skepticism among Americans already hesitant to get the shots.

“There’s risks on either side,” Fauci was quoted as saying by the Washington Post in a report published late on Monday.

“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.”

He added that he spoke with UK health officials on Monday who have opted to delay second doses to maximize giving more people shots more quickly. Fauci said that strategy would not make sense in the United States.

He said the science does not support delaying a second dose for those vaccines, citing research that a two-shot regimen creates enough protection to help fend off variants of the coronavirus that are more transmissible, whereas a single shot could leave Americans at risk from variants such as the one first detected in South Africa.

“You don’t know how durable that protection is,” he said.

Fauci said on Sunday he was encouraging Americans to accept any of the three available COVID-19 vaccines, including the newly approved Johnson & Johnson shot.

The US government authorized Johnson & Johnson’s single-dose COVID-19 vaccine on Saturday, making it the third to be available in the country following the ones from Pfizer/BioNTech and Moderna that require two doses.

COVID-19 has claimed more than half a million lives in the United States, and states are clamoring for more doses to stem cases, hospitalizations and deaths.

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