An international group of scientists, including top leaders at the World Health Organization and two departing Food and Drug Administration officials, said in a paper published Monday that booster doses of coronavirus vaccines are not yet needed for the general public.
Writing in the medical journal The Lancet, the scientists argue that current evidence shows fully vaccinated individuals remain highly protected against severe COVID-19, and make the case that additional doses would be better used in people who remain unvaccinated.
"The vaccines that are currently available are safe, effective, and save lives," the scientists wrote. "The limited supply of these vaccines will save the most lives if made available to people who are at appreciable risk of serious disease and have not yet received any vaccine."
The paper's publication comes as the Biden administration is pushing forward with plans to roll out booster doses widely in the U.S., and as some countries in Europe advance similar efforts. But those decisions have become controversial, particularly as many nations globally are still struggling to secure enough supplies for initial vaccinations for their citizens.
The Biden administration has also been criticized for getting ahead of the scientific decisionmaking process at the FDA and the Centers for Disease Control and Prevention. Disagreement over the White House's plans, announced by Biden in mid-August, reportedly played a role in the unexpected retirements of the FDA's Marion Gruber and Philip Krause, both top vaccine reviewers at the agency who will depart later this fall.
Gruber and Krause are co-authors of the Lancet paper, making the arguments in it a public rebuttal of the administration's plans. In addition to the two FDA officials, the paper's authors include WHO Chief Scientist Soumya Swaminathan and Michael Ryan, executive director of the WHO's Health Emergencies Program.
The WHO has called for a "moratorium" on plans to administer booster doses widely, at least until the end of this year.
In the U.S., the FDA has authorized booster doses of either Pfizer's or Moderna's vaccines for people who are immunocompromised and the initial two-dose series is insufficient to establish protection against COVID-19. On Friday, however, advisers to the agency will meet to discuss widening that authorization, at least for Pfizer's vaccine, to the broader population.
The Biden administration previously set Sept. 20 as its target date to begin rolling out booster doses beyond immunocompromised individuals.
Behind the booster debate is a scramble by scientists worldwide to determine how long the high protection offered by several of the available vaccines lasts, particularly against more transmissible variants like delta. While the effectiveness of vaccines like Pfizer's and Moderna's was established in randomized clinical trials, much of the recent evidence on their durability has come from studies of their use in the real world, which may be confounded by a wide range of factors.
"Randomized trials are relatively easy to interpret reliably, but there are substantial challenges in estimating vaccine efficacy from observational studies undertaken in the context of rapid vaccine roll-out," the scientists wrote in their Lancet paper.
While it's clear that vaccine protection remains high against severe coronavirus disease, there's more uncertainty about how well that protection holds up against coronavirus infection and more mild cases. Accumulating evidence indicates it's likely lower, but it's unclear by exactly how much and whether any reduction would eventually compromise efficacy versus severe disease as well.
Data from Israel, which vaccinated large swathes of its population relatively quickly, has suggested a more rapid decline, prompting that country to start administering a third dose and even consider a fourth.
Those conclusions, however, have mostly been based on measurements of the levels of antibodies against the coronavirus found in vaccinated individuals' blood. Scientists have not yet established a threshold for protection based on antibody levels, and other components of the immune system, like T cells, may play a greater role in long-term defense against COVID-19.
"Even if humoral immunity appears to wane, reductions in neutralizing antibody titre do not necessarily predict reductions in vaccine efficacy over time, and reductions in vaccine efficacy against mild disease do not necessarily predict reductions in the (typically higher) efficacy against severe disease," the scientists wrote in the Lancet.
The group also noted concerns around the impact widespread administration of boosters might have on public messaging around vaccination, and on the potential for additional side effects to a third dose.
Ultimately, however, the scientists' argument is centered on the benefit that could be gained from initial vaccinations of a greater number of people worldwide, particularly as supplies remain limited outside of the richest countries.
"Even if boosting were eventually shown to decrease the medium-term risk of serious disease, current vaccine supplies could save more lives if used in previously unvaccinated populations than if used as boosters in vaccinated populations," they wrote.