COVID-19 vaccine booster shots will not be necessary until mRNA vaccines become less effective, federal health officials say. But immunocompromised Americans are on their radar as potential near-term recipients, they told U.S. senators on Tuesday.

Representatives from the Food and Drug Administration, the Centers for Disease Control and Prevention and the National Institutes of Health revealed their thinking — if not their plans — regarding COVID-19 booster shots in testimony before a Senate Health Committee. With the newer and highly transmissible SARS-CoV-2 delta variant now accounting for 83% of COVID-19 cases in the general population, some senators pressed the health officials for a solid timeline on when Americans should receive protective additional shots. 

Sen. Mitt Romney (R-Utah), for example, asked why boosters weren’t at least being given to the elderly and immunocompromised, as Israel has done.

A special authorization will be needed for boosters, since the original vaccines are currently approved under emergency use authorization only, FDA acting chief Janet Woodcock said, according to ABC News. Health officials said they also were waiting for Pfizer to deliver its clinical data on booster shots.

In the meantime, a CDC advisory committee set to meet Thursday will consider data and clinical considerations for administering additional doses to immunocompromised people. 

But Woodcock said booster shots should be given only when necessary to the general population, ABC News reported.

“We can’t just boost them all the time, right? We need to boost when it’s appropriate,” she told the senators.

A booster shot campaign will make sense when mRNA vaccines from Pfizer and Moderna wane from their current level of 93% or greater efficacy to between 70% and 80%, added Anthony S. Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases. This is happening gradually as new variants such as delta reduce vaccine efficacy, he said.

CDC  Director Rochelle Walensky, M.D., said that her agency is following the health of thousands of nursing home residents, healthcare workers and essential workers, and testing them weekly to help make decisions about boosters, among other COVID-19-related concerns, ABC reported. 

J&J COVID-19 vaccine may require a booster

In the meantime, a yet-to-be-reviewed study suggests that people who received the Johnson & Johnson COVID-19 vaccine may indeed need a booster to improve their immunity to new virus variants.

The mRNA vaccines made by Pfizer-BioNTech and Moderna may better protect against the newer and highly contagious delta and lambda strains, at 94% to 95% efficacy, than the Johnson & Johnson vaccine, which is approximately 67% effective, investigators reported.

“The message that we wanted to give was not that people shouldn’t get the J&J vaccine, but we hope that in the future, it will be boosted with either another dose of J&J or a boost with Pfizer or Moderna,” study lead Nathaniel Landau, a virologist at NYU’s Grossman School of Medicine, told The New York Times. 

Other experts said that the study’s results are not surprising since all vaccines appear to work better with two doses, the Times reported. “I have always thought, and often said, that the J&J vaccine is a two-dose vaccine,” John Moore, a virus expert at Weill Cornell Medicine in New York, told the news outlet.

The study, the results of which could be challenged in peer review and must still be vetted for publication, was published on the preprint server BioRxiv.