But since Israel recently began its second booster program, researchers have not been able to determine if the added protection is short-lived. Israel began providing fourth doses to health care workers in late December, and quickly expanded eligibility to include 60 and older and other vulnerable groups.

second study, of Israeli healthcare workers, showed that although IV Pfizer or Moderna vaccines boosted antibody levels, they were not very effective in preventing infection. These findings underscore the urgency of developing vaccines that target any circulating variant, the researchers said.

The US National Institutes of Health and several vaccine manufacturers are studying how to update vaccines. A federal health official said results are not expected until the summer.

Some senior administration officials say that based on the evidence, a second boost could now make sense for older Americans, but not for the general population. The US Food and Drug Administration is expected to hold a meeting of the expert advisory committee next month to discuss the issue of the fourth shot. The developments were at the request of Pfizer Earlier reported by The Washington Post.

Last month he was asked if everyone needed another injection, Dr. Peter Marks, the US Food and Drug Administration’s largest vaccine regulator, saidBarring any surprises from the new variables, perhaps the best thing would be to think about our boosting strategy in conjunction with next fall’s flu shot, boosting as many people as possible at that time.” Dr. Anthony S. Fauci, the White House’s chief medical advisor, suggested that any move toward a second booster now would likely target those most at risk, possibly based on age and underlying conditions.

To date, about two-thirds of Americans 5 years of age and older have been fully vaccinated with two doses of the vaccine. Only about half of those eligible for the boosters got it, but the percentage goes up to two-thirds for those 65 and older, according to the Centers for Disease Control and Prevention.

In a call with reporters Tuesday, senior administration officials said the administration is short of funds for new doses. One said the administration has enough supplies to administer shot four for people 65 and older, but that it can’t extend that effort to everyone without more funding from Congress.