Health experts voice support for opening booster access to older and at-risk Americans.

American health experts spoke out on social media and other platforms on Friday and Saturday to applaud the likelihood that federal regulators could soon make Pfizer-BioNTech booster shots available for those 65 and older or at high risk of severe Covid-19, though some argued that the cutoff age should be lower.

The experts were reacting to the actions on Friday of an advisory committee to the Food and Drug Administration. That panel’s recommendation that the boosters be authorized was paired with its rejection of moving toward a blanket authorization for everyone 16 and older.

The only Covid-19 vaccine for which the F.D.A. has enough information to determine a booster shot’s effectiveness is the one made by Pfizer-BioNTech. The timing for the other two vaccines in use in the United States, Moderna’s and Johnson & Johnson’s, is murky.

But Dr. Jha also noted in a separate tweet that boosters would benefit those 60 and above.

Dr. Eric Topol, a professor of molecular medicine at Scripps Research in La Jolla, Calif., called the F.D.A. panel’s decision “a very good outcome.” The decision, he wrote on Twitter, recognized “the need for high-risk individuals, due to co-existing conditions or occupational exposures, such as the heathcare workforce, essential workers, and teachers.”

However, Dr. Topol also questioned why the cutoff point for third-shot eligibility was set at 65, rather than including people 60 or older. He also said the updated F.D.A. advisory “didn’t address the vulnerability of people who received J&J shots.”

Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia, told The Associated Press that he supported a third dose for older adults but that “I really have trouble” when it comes to getting behind the idea of administering the shot to anyone closer to the age of 16. “The question becomes what will be the impact of that on the arc of the pandemic, which may not be all that much,” Dr. Offit said.

For some Americans outside the medical or scientific fields, the F.D.A. panel’s recommendations seemed opaque, or even contradictory.

For instance, Jen Macy, a wedding and event floral designer in Orange County, Calif., tweeted and also discussed with a Times reporter what she felt was an urgent question: “Can you explain why boosters are being recommended for high risk people but the general population needs to wait for further testing? This makes no sense.”

Michael Knowles, a conservative media figure with a large following on social media and via The Daily Wire, a site that publishes commentary and podcasts, took a comic dig at the F.D.A. panel’s actions. “The vaccines are so effective that you need a booster and so safe that the FDA won’t approve the booster,” he wrote on Twitter. “Do I have that right?”