The U.S. Food and Drug Administration is leaning toward authorizing half-dose booster shots of the Moderna Inc. coronavirus vaccine, satisfied that it’s effective in shoring up protection, people familiar with the matter said.

The authorization would set the stage to further widen the U.S. booster campaign after earlier authorization of the Pfizer Inc.-BioNTech shot. About 170 million fully vaccinated people in the U.S. received the Moderna or Pfizer shots, or 92 percent of the total inoculated so far.

The people spoke on the condition of anonymity, before a potential announcement. It’s not clear when an announcement will come.

Any authorization would also introduce different dosage levels for boosters. Moderna’s initial inoculations contained 100-microgram doses, and the company’s submission to regulators amounted to a push to authorize a half-dose booster.

Pfizer’s shot, for comparison, has 30-microgram initial doses and a 30-microgram booster. Boosters are so far being given with, or planned for, the same vaccine that a person received initially, though studies are ongoing about whether to mix vaccines.

Proceeding with a 50-microgram dose could reduce the risk of side effects from a booster, and would also allow Moderna to produce more doses globally in the near term. That would ease supply constraints and potentially blunt criticisms of rich countries beginning sprawling booster campaigns before many nations have given widespread first shots.

Moderna declined to comment on Tuesday night. The White House and the FDA declined to comment.

The U.S. is rolling out boosters to head off what President Biden’s health advisers warn are a pair of concerning trends: Hints that vaccine efficacy wanes over months, and that the two-dose regimens are weaker generally against the delta variant than against other iterations of the virus.

The U.S. has dealt with a summer and fall wave of new cases, hospitalizations and deaths, driven by spread among unvaccinated people but increasing the exposure risk for the vaccinated.

The booster campaign was widely expanded late last week when Centers for Disease Control and Prevention Director Rochelle Walensky overruled an advisory panel to broaden eligibility for Pfizer’s booster shots. The World Health Organization has called for a moratorium on boosters this year, a request the U.S. has disregarded.

As of now, only people who received Pfizer shots are eligible for a booster in the U.S. “We will continue to evaluate data as it becomes available in real time and with urgency, and update our recommendations to make sure that all of those at risk have the protection they need,” Walensky said Tuesday.

The U.S. has donated about 160 million shots abroad, and the vast majority were surplus domestic supply from Moderna. Biden has also announced deals for a total of 1 billion Pfizer shots bought specifically for donation. Those began shipping in August and will be delivered by September 2022.

The FDA had been seeking information about the effectiveness of a full third dose of the Moderna vaccine, but is now ready to move forward and consider the half-dose booster Moderna has proposed, the people said.

Biden, who got his Pfizer booster on Monday, has said this remains a pandemic of the unvaccinated.

Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases, has said he believes Pfizer and Moderna will eventually be considered three-dose vaccines.

As the vaccination campaign widens, sites that administer them will have to juggle different versions. In addition to Moderna potentially adding a half-dose booster, Pfizer is seeking authorization of a vaccine for children ages 5 to 11, with a 10-microgram dose – one-third the strength given to those 12 and up.

Fauci has indicated that there’ll be progress soon on booster shots for Moderna as well as Johnson & Johnson’s one-dose vaccine. “I believe it will be weeks and not months,” he told NBC’s “Meet the Press” earlier this month.

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