Come this fall, only older Americans and those with chronic health problems may be urged to get the COVID shot — assuming the vaccine is available at all.

For years now, scientific advisers to the Centers for Disease Control and Prevention have been debating whether to continue to recommend that Americans 6 months of age and older be immunized, or to switch to a so-called risk-based strategy targeting only the most vulnerable, as is the practice now in most other countries.

The advisers are expected to decide on a way forward at a meeting in June. But the debate may have become irrelevant. New requirements for clinical testing of vaccines, announced this month, may delay the availability of shots that had formerly been updated annually without complicated trials.

“Substantial updates to existing vaccines — such as those related to seasonal strain changes or antigenic drift — may qualify as ‘new products’ and therefore require additional clinical evaluation,” the Department of Health and Human Services said in a statement.

That category includes the COVID shots and “may” even include the seasonal flu vaccine, according to Andrew Nixon, a spokesperson for HHS.The Food and Drug Administration has already asked Novavax for more clinical data before approving its COVID vaccine. (Currently it has emergency use authorization, not full approval, for people ages 12 and older.)

“We want to see vaccines that are available for high-risk individuals, and at the same time we want some good science, we want some good clinical data,” Dr. Marty Makary, who heads the agency, told reporters at a conference Thursday.

Officials in Washington have also been asking pointed questions of CDC scientists about COVID’s toll on children younger than 12, an indication that they may be considering an end to the use of the vaccine in that age group, according to an official who did not wish to be identified for fear of retribution.

The Wall Street Journal reported Thursday that HHS plans to stop recommending the COVID vaccine for children and pregnant women altogether, citing people familiar with the matter.

In May 2021, Robert F. Kennedy Jr. sought to rescind authorization for the shots in a petition to the FDA. He has said, falsely, that the vaccines had killed more people than the virus.

The leaders he has installed at the agencies he leads, including Makary, have also been outspoken critics of annual COVID shots, particularly for children.

On the latter point, many public health experts agree, including some advisers to federal agencies. They have suggested for years now that annual booster shots might offer only marginal benefits to relatively younger, healthy Americans.

“I think what Dr. Makary is saying is that if everybody already has immunity to the virus, then maybe the vaccine doesn’t add that much, or maybe it adds something only in certain subpopulations,” said Dr. Philip Krause, a former FDA vaccine regulator. “I think it’s a reasonable question to want to know the answer to.”

(Krause resigned from the FDA in the fall of 2021 because he felt there was not enough evidence to support COVID booster shots for adults.)

The CDC already collects data on the benefits of the COVID vaccines, which have decreased over time as more Americans gain greater immunity to the coronavirus.

But the vaccine should remain available to some people, including adults 75 and older, who remain at high risk, Krause and other experts noted.

“In my opinion, the data from the CDC provide pretty strong evidence that there are some people who are still benefiting quite a bit from the vaccines,” he said. “There are still more COVID deaths than we would like.”

Kennedy has falsely asserted that none of the vaccines routinely offered to children have been tested in placebo clinical trials, prompting sharp rebuttals from several groups of scientists, and from Sen. Bill Cassidy, R-La., whose vote cinched Kennedy’s nomination as health secretary earlier this year.

Some experts fear that the new federal limits are just the first steps in a campaign to undermine the use of all vaccines, including those in the childhood immunization schedule, by raising doubts about their safety and imposing requirements that make it untenable for companies to keep making them.

“I think that we are in the midst of watching the vaccine infrastructure being torn down bit by bit,” said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and an adviser to the FDA.

Tara Smith, an epidemiologist at Kent State University College of Public Health who tracks the anti-vaccine movement, said there was jubilation among some groups that all vaccines, including the decades-old flu vaccine, might be challenged.

“I think everything is a target,” she said.

COVID fatalities have plummeted each year since the start of the pandemic, although there were still roughly 1,000 deaths per week during last winter’s peak. The vast majority occurred among adults ages 75 and older.

The FDA has not elaborated on what additional testing of COVID vaccines might entail. But officials may offer some clarity before the agency’s scientific advisers meet next week to discuss vaccines for the fall.

It would be unethical to offer a placebo to participants in a randomized trial if the virus still poses risks. And the results of any new trials would not be available in time for the fall, experts noted.

“In order to create new requirements for vaccines, there would have to be evidence that that would have any benefit for the public,” said Dr. Eric Rubin, one of the FDA’s advisers and editor-in-chief of the New England Journal of Medicine.

“The issue, of course, is when you go into this having decided what the answer is, rather than looking at the data objectively,” he said.

Some of the CDC’s advisers had mixed feelings even about moving to risk-based vaccine recommendations.

Such guidelines make sense in countries with universal health coverage, they said.

But in the United States, insurance companies are not required to pay for an immunization that is not recommended by the CDC.

That may leave a vaccine that is too expensive for some who need it and are not explicitly included in a risk-based recommendation. A blanket recommendation may be more effective, some experts said.