WASHINGTON — With many Americans who got Pfizer vaccinations already rolling up their sleeves for a booster shot, millions of others who received the Moderna or Johnson & Johnson vaccine wait anxiously to learn when it’s their turn.

Federal regulators begin tackling that question this week.

On Thursday and Friday, the Food and Drug Administration convenes its independent advisers for the first stage in the process of deciding whether extra doses of the two vaccines should be dispensed and, if so, who should get them and when. The final go-ahead is not expected for at least another week.

After the FDA advisers give their recommendation, the agency itself will make a decision on whether to authorize boosters. Then next week, a panel convened by the Centers for Disease Control and Prevention will offer more specifics on who should get them. Its decision is subject to approval by the CDC director.

The process is meant to bolster public confidence in the vaccines.

But it has already led to conflicts among experts and agencies — and documents the FDA released Tuesday suggest this week’s decisions will be equally difficult.

In one earlier vaccine dispute, the CDC’s advisory panel last month backed Pfizer boosters at the six-month point for older Americans, nursing home residents and people with underlying health problems.

But CDC Director Dr. Rochelle Walensky overruled her advisers and decided boosters should also be offered to those with high-risk jobs such as teachers and health care workers, adding tens of millions more Americans to the list.

Some health experts fear the back-and-forth deliberations are muddling the public effort to persuade the unvaccinated to get their first shots. They worry that the talk of boosters will lead people to wrongly doubt the effectiveness of the vaccines in the first place.

When the FDA’s panel meets to review the Moderna and J&J vaccines, experts will discuss whether a third Moderna shot should contain just half the original dose and what’s the best timing for a second shot of the single-dose J&J vaccine.

The panel will also look into the safety and effectiveness of mixing-and-matching different brands of vaccine, something regulators have not endorsed so far.

An estimated 103 million Americans are fully vaccinated with Pfizer’s formula, 69 million with Moderna’s and 15 million with J&J’s, according to the CDC. Regulators took up the question of Pfizer boosters first because the company submitted its data ahead of the other vaccine makers.

Tim Anderson, a U.S. history teacher at a high school outside Louisville, Kentucky, already had his two Moderna shots months before he came down with COVID-19 in August. While his symptoms hit him “like a sledgehammer,” he is convinced that the inoculation saved him and his girlfriend from the more severe effects of the disease.

The two are now awaiting clearance of a Moderna booster shot.

“Until we can build up enough immunity within our own self and, you know, as a group of humans, I’m willing to do what I need to do,” Anderson, 58, said.

The FDA meetings come as U.S. vaccinations have climbed back above 1 million per day on average, an increase of more than 50% over the past two weeks. The rise has been driven mainly by Pfizer boosters and employer vaccine mandates.

While the FDA and CDC so far have endorsed Pfizer boosters for specific groups only, Biden administration officials, including Dr. Anthony Fauci, have suggested that extra shots will eventually be recommended for most Americans.

The two initial Moderna shots contain 100 micrograms of vaccine each. But the drugmaker says 50 micrograms ought to be enough for a booster for healthy people.

A company study of 344 people gave them a 50-microgram shot six months after their second dose, and levels of virus-fighting antibodies jumped.

Moderna said the booster even triggered a 42-fold rise in antibodies able to target the extra-contagious delta variant.

Side effects were similar to the fevers and aches that Moderna recipients commonly experience after their second regular shot, the company said.

As for people who got the J&J vaccine, the company submitted data to the FDA for different options: a booster shot at two months or at six months. The company did not signal its preference.