Many Massachusetts residents will likely soon be able to schedule appointments for a second booster shot against COVID-19 after federal health regulators on Tuesday endorsed an additional dose in older adults and those who are immunocompromised, though the timing of the Bay State rollout remains unknown.

As the Baker administration winds down state-sponsored testing options with demand dwindling and the virus's impact well below the winter surge, state officials are gearing up to make additional immunization available to a large swath of the population.

The U.S. Food and Drug Administration announced Tuesday that it backs offering a second Moderna or Pfizer booster dose to adults 50 years old and above and some immunocompromised people who have already completed their primary vaccination series and received an initial booster at least four months earlier.

Hours later, the U.S. Centers for Disease Control followed suit, updating its recommendations to allow older adults and those who are immunocompromised to get a second booster at least four months after their initial booster.

It was not immediately clear Tuesday afternoon when the opening salvo of extra booster shots would occur in Massachusetts.

A Baker administration spokesperson who would only communicate on background said Tuesday afternoon that the state is waiting on the green light from the CDC to make second boosters available to older adults. The spokesperson did not respond to a follow-up inquiry seeking clarification about what additional action the federal agency still needs to take.

Massachusetts has hundreds of locations offering vaccines and boosters, the spokesperson said, adding that the state is "well prepared" to handle the demand of second boosters.

Once the additional shots roll out, adults 50 and older and eligible residents who are immunocompromised will be able to book appointments the same way they did for their original doses and first boosters. Slots will be posted online on the state's vaxfinder.mass.gov website, and many providers also offer their own web platforms to book appointments.

Federal regulators determined the benefits of a second booster dose would outweigh any potential risks, with the FDA pointing to studies of both the Pfizer and Moderna shots that it said found "no new safety concerns."

"Current evidence suggests some waning of protection over time against serious outcomes from COVID-19 in older and immunocompromised individuals. Based on an analysis of emerging data, a second booster dose of either the Pfizer-BioNTech or Moderna COVID-19 vaccine could help increase protection levels for these higher-risk individuals," said FDA Center for Biologics Evaluation and Research Director Peter Marks. "Additionally, the data show that an initial booster dose is critical in helping to protect all adults from the potentially severe outcomes of COVID-19."

Public health experts have pitched booster shots as a critical tool in keeping COVID-19 at bay, arguing that the base vaccines do not offer enough protection against the virus's variants.

While Massachusetts has one of the highest overall vaccination rates in the country, only about 2.9 million Bay Staters have received a booster shot as of Tuesday, representing roughly 55 percent of the more than 5.3 million who completed both Moderna or Pfizer primary shots or received the single-shot Johnson & Johnson vaccine.

The push to get second boosters into arms comes with case counts and hospitalizations for COVID-19 well below their wintertime, omicron-fueled peak. As of Sunday, the seven-day average of newly confirmed infections in Massachusetts stood at 782 per day, down from more than 22,000 per day in early January.

Elected officials have lifted some of their pandemic response measures in recent months, including a K-12 school mask mandate, a proof-of-vaccine requirement to solicit many indoor Boston businesses, and the nearly two-year closure of the State House.

On March 4, the Baker administration announced it would close down 30 of the state-run Stop the Spread free testing sites effective April 1, leaving a total of 11 running through at least May 15.

Officials cited "a significant decline in demand" for the sites, where residents can get tested for free without insurance or ID information required, as well as the availability of at-home rapid antigen tests as justification for the downsizing.

Since the beginning of January, the administration said, the state's Stop the Spread sites have seen an 80 percent decrease in test volumes.

The 11 sites that will remain online -- two in Lawrence, two in Springfield and one each in Everett, Framingham, Lynn, New Bedford, Randolph, Revere and Worcester -- collectively accounted for nearly three-quarters of all tests administered across the state-run program between Jan. 20 and Feb. 16, according to the Baker administration.

Berkshire County lawmakers criticized the administration's decision, unsuccessfully urging Health and Human Services Secretary Marylou Sudders to keep at least one testing location open in their county so residents would not need to drive roughly an hour to Springfield to access the closest free, state-run site, the Berkshire Eagle reported.

Cities and towns where Stop the Spread sites will go dark later this week will each receive a chunk of at-home rapid tests, ranging from 1,080 to 6,120, to distribute to residents. Those tests came from a pool of 26 million the administration purchased in January.

In total, Massachusetts has more than 450 locations where residents can get tested, many of which require appointments.

The federal government allows Americans to order rapid tests online and now requires health insurers to cover eight at-home rapid tests per covered person each month, but advocates warn that framework falls short of protecting the uninsured.