WASHINGTON (AP) — The U.S. government on Friday moved to open up COVID-19 booster shots to all adults, expanding efforts to get ahead of rising coronavirus cases that experts fear could snowball into a winter surge as millions of Americans travel for the holidays.

The Food and Drug Administration's decision stands to simplify what has been a confusing list of who's eligible for a booster: Now, anyone 18 or older can choose either a Pfizer or Moderna booster six months after their last dose, regardless of which vaccine they had first. The move came after about a dozen states had started offering boosters to all adults on their own.

"We heard loud and clear that people needed something simpler — and this, I think, is simple," FDA vaccine chief Dr. Peter Marks told The Associated Press.

But there's one more step before that policy is final: The Centers for Disease Control and Prevention must agree. Its scientific advisers supported the move Friday afternoon after discussing the safety and usefulness of Pfizer and Moderna boosters in even healthy young adults.

The CDC's advisers said anyone 18 and older can choose a booster of the Pfizer or Moderna vaccine but went an extra step and stressed that people 50 and older should get one. A final CDC decision was expected later Friday.

"It's a stronger recommendation," said CDC adviser Dr. Matthew Daley of Kaiser Permanente Colorado. "I want to make sure we provide as much protection as we can."

The No. 1 priority still is getting more unvaccinated Americans their first doses. That's because all three COVID-19 vaccines used in the U.S. continue to offer strong protection against severe illness, including hospitalization and death, without a booster. But protection against infection can wane with time.

"Death from COVID-19 is for most people living in the United States vaccine-preventable," Daley noted.

But if the CDC agrees, tens of millions more Americans who are six months past their last Pfizer or Moderna shot could get an extra dose of protection before the new year. The Moderna booster comes as half the dose of earlier shots. Anyone who got the one-dose Johnson & Johnson vaccine already can get a booster after two months.

Teen boosters aren't yet under discussion, and kid-sized doses of Pfizer's vaccine are just now rolling out to children ages 5 to 11.

The push to expand boosters comes as new COVID-19 cases have climbed steadily over the last three weeks, especially in states where colder weather is driving people indoors. Some states didn't wait for federal officials to act and opened boosters to all adults.

Marks said he understood why some governors got out ahead of the FDA.

"We're going into a cold season, cases going up, high travel season, people indoors sharing good holiday times together," he said. "They probably saw the specter of what could happen here, and were trying — well intentioned — to do something."

Boosters for everyone was the Biden administration's original goal. But until now, U.S. health authorities — backed by their scientific advisers — have questioned the need for such widespread boosters. Instead, they endorsed Pfizer or Moderna boosters only for vulnerable groups such as older Americans or those at high risk of COVID-19 because of health problems, their jobs or their living conditions.

This time around, the FDA concluded the overall benefits of added protection from a third dose for any adult outweighed risks of rare side effects from Moderna's or Pfizer's vaccine, such as a type of heart inflammation seen mostly in young men.

Several other countries have discouraged use of the Moderna vaccine in young people because of that concern, citing data suggesting the rare side effect may occur slightly more with that vaccine than its competitor.

Pfizer told CDC's advisers that in a booster study of 10,000 people as young as 16, there were no more serious side effects from a third vaccine dose than earlier ones. That study found a booster restored protection against symptomatic infections to about 95% even while the extra-contagious delta variant was surging.

Britain recently released real-world data showing the same jump in protection once it began offering boosters to middle-aged and older adults, and Israel has credited widespread boosters for helping to beat back another wave of the virus.

More than 195 million Americans are fully vaccinated, defined as having received two doses of the Pfizer or Moderna vaccines or the single-dose J&J. More than 30 million already have received a booster. That includes some people who weren't eligible; many vaccine sites weren't checking qualifications.

Some experts worry that all the attention to boosters may harm efforts to reach the 60 million Americans who are eligible for vaccinations but haven't gotten the shots. There's also growing concern that rich countries are offering widespread boosters when poor countries haven't been able to vaccinate more than a small fraction of their populations.

"In terms of the No. 1 priority for reducing transmission in this country and throughout the world, this remains getting people their first vaccine series," said Dr. David Dowdy of Johns Hopkins Bloomberg School of Public Health.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute's Department of Science Education. The AP is solely responsible for all content.