COVID-19 vaccine booster shots might not be needed for most people, according to a large international review.
The review — conducted by a team that included scientists from the World Health Organization and the U.S. Food and Drug Administration — concluded that current vaccines are effective enough against severe COVID-19, even from the Delta variant, and that booster shots are unnecessary.
The findings, published Sept. 13 in The Lancet, are based on a review of all available published literature and results of clinical trials.
"The vaccines that are currently available are safe, effective, and save lives," said study co-author Dr. Soumya Swaminathan, chief scientist at the WHO.
"Although the idea of further reducing the number of COVID-19 cases by enhancing immunity in vaccinated people is appealing, any decision to do so should be evidence-based and consider the benefits and risks for individuals and society," she said in a journal news release. "These high-stakes decisions should be based on robust evidence and international scientific discussion."
According to the review, vaccines were 95% effective against severe disease both from the Delta and Alpha variant, and more than 80% effective at protecting against infection from these variants.
Vaccines are less effective against asymptomatic disease or transmission than against severe disease, according to the review. It added that unvaccinated people are the major drivers of transmission and are at the greatest risk of severe disease.
"Taken as a whole, the currently available studies do not provide credible evidence of substantially declining protection against severe disease, which is the primary goal of vaccination," said lead author Dr. Ana Maria Henao-Restrepo, a medical officer at the WHO's Initiative for Vaccine Research.
She said the limited supply of vaccine will save the most lives if made available to unvaccinated people who are at risk of serious disease.
"Even if some gain can ultimately be obtained from boosting, it will not outweigh the benefits of providing initial protection to the unvaccinated," Henao-Restrepo 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."
Although antibodies in vaccinated people wane over time, the authors noted that it does not predict that a lack of protection against severe disease.
Protection against severe disease is not only from antibody responses, which might be short-lived for some vaccines, but also by memory responses and cell immunity, which are longer-lived, the researchers explained. If boosters are needed, they would be in circumstances where the benefits outweigh the risks.
Even without any loss of effectiveness, however, increasing success in delivering vaccines to large populations will lead to increasing more widespread immunization, the researchers said. As a result, more cases that do occur would be less severe breakthrough infections, especially if vaccination leads people to change their behavior.
But, they added, the ability of vaccines to elicit an antibody response against current variants indicates that these variants have not yet evolved so much they are likely to escape the protection by the vaccines.
If boosters are needed, the researchers said, they would most likely be developed against specific variants not covered by current vaccines. This strategy is like that for flu shots, which changes as flu strains evolve from year to year.