But since Israel only recently began its second booster program, researchers could not determine whether the added protection was short-lived. Israel began offering fourth doses to health care workers in late December, then quickly broadened eligibility to those 60 and older and other vulnerable groups.
The second study, of Israeli health care workers, showed that even though fourth shots of either Pfizer’s or Moderna’s vaccine boosted antibody levels, they were not very effective at preventing infections. Researchers said those findings underscored the urgency of developing vaccines that target whatever variant is circulating.
The National Institutes of Health in the United States and various vaccine manufacturers have been studying how the vaccines could be updated. One federal health official said results were not expected until the summer.
Some senior administration officials say that depending on the evidence, a second booster could make sense now for older Americans, but not for the general population. The F.D.A. is expected to convene a meeting of its expert advisory committee next month to discuss the issue of fourth shots. Developments on Pfizer’s request were reported earlier by The Washington Post.
Asked last month whether everyone would need another injection, Dr. Peter Marks, the F.D.A.’s top vaccine regulator, said, “Barring any surprises from new variants, maybe the best thing is to think about our booster strategy in conjunction with the influenza vaccine next fall, and get as many people as possible boosted then.” Dr. Anthony S. Fauci, the chief medical adviser to the White House, has suggested that any move toward a second booster now would most likely be aimed at those most at risk, possibly based on age and underlying conditions.
To date, about two-thirds of Americans 5 and older have been fully vaccinated with two shots of a vaccine. Only about half of those eligible for boosters have received them, but the proportion rises to two-thirds for those 65 and older, according to the Centers for Disease Control and Prevention.
In a call with reporters on Tuesday, senior administration officials said the administration was running short on funds for new doses. The administration has enough supply to manage a fourth shot for people 65 and older, one said, but could not expand that effort to everyone without more funding from Congress.