Moderna refiled its application to the Food and Drug Administration on Wednesday to approve vaccine booster doses for all adults ages 18 and older.

FDA authorization would open the doors for tens of millions of Americans to receive a third shot of the vaccine, as a growing amount of data demonstrates that vaccine efficacy wanes over time. The FDA and Centers for Disease Control and Prevention cleared the shots last month for everyone 65 and older as well as adults who are at a higher risk of developing severe Covid or who work or live in settings that places them at greater risk of catching the virus.

Moderna’s application, which was originally submitted in September, comes as the FDA is expected to authorize Pfizer booster shots for all adults as early as this week. If the CDC gives it the OK, Pfizer’s third shots could be administered as early as this weekend to adults who’ve completed their first two rounds of shots at least six months ago.

Pfizer’s boosters, like Moderna’s, were previously authorized for the elderly and those people at high-risk. People ages 18 and older who have received Johnson & Jonson’s single-dose vaccine are already eligible for a booster.

CDC Director Rochelle Walensky said Wednesday that the public health agency plans to “act swiftly” on booster doses as soon as the FDA reviews the data and provides authorization.

Broadly administering booster doses for all adults is controversial among public health authorities. The World Health Organization has repeatedly criticized wealthy nations for moving forward with third shots as many people in poorer nations still have limited access to vaccines.

The WHO has warned that failing to push for broad access to vaccines in the developing world could lead to future variants of the virus that would then undermine protection in countries that have broadly immunized their populations, threatening what progress has been made.

Booster doses have also stirred some controversy in the U.S., as more than 60 million Americans remain unvaccinated. White House chief medical advisor Dr. Anthony Fauci on Wednesday not only encouraged Americans to get vaccinated, but to receive their boosters if they are eligible.

“It is not too late. Get vaccinated now. And importantly, if you are already vaccinated six months or more ago and eligible for a boost, get a boost,” Fauci said. “The Israelis have shown that when you boost, you multifold diminish the likelihood of getting infected, getting sick, or dying.”

The U.S. has administered more than 168 million doses of Moderna’s vaccine and more than 257 million doses of Pfizer’s shot. Health officials have administered more than 16 million doses of Johnson & Johnson’s vaccine.

Nearly 80% of Americans ages 12 and older have received at least one dose of the three vaccines, according to CDC data. More than 30.7 million people have already received their booster shots in the U.S., the data shows.

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