Widespread use of COVID-19 booster vaccination is not supported by the available evidence in populations where initial vaccination is already effective , according to experts from the Food and Drug Administration and the World Health Organization, among others.

“If boosters are ultimately to be used, there will be a need to identify specific circumstances in which the direct and indirect benefits of doing so are, on balance, clearly beneficial,” wrote the FDA’s Marion Gruber, Ph.D., Philip Krause, M.D., and colleagues in a viewpoint published Monday in the Lancet.

Boosting may be appropriate in some populations, such as immunocompromised patients as defined in recent recommendations from the Centers for Disease Control and Prevention, the authors wrote. But even in those cases, it is unclear whether an additional dose of the same vaccine or of a different vaccine would be more beneficial, they noted.

The question of what an effective dose might be also remains unanswered, they wrote. “[G]iven the robust booster responses reported for other vaccines, adequate booster responses might be achievable at lower doses, potentially with reduced safety concerns.”

COVID-19 vaccine supplies could save more lives if used in previously unvaccinated populations than if used as boosters in vaccinated populations — even if booster vaccines are eventually shown to decrease the risk of serious disease, the authors wrote. While their efficacy could wane in the future, the current vaccines continue to be effective against severe disease, including that caused by the delta variant, they added.

“If vaccines are deployed where they would do the most good, they could hasten the end of the pandemic by inhibiting further evolution of variants,” they wrote.

But if a booster vaccination campaign is rolled out without solid scientific justification, this could have an adverse effect on public confidence in vaccines and “undermine messaging about the value of primary vaccination,” they argued.

Krause and Gruber recently announced that they would be departing the FDA, a decision due at least in part to their disapproval of the Biden administration’s handling of booster decision making and messaging to date, according to Endpoints News.