At a Wednesday meeting of the Food and Drug Administration’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) of outside advisers, members warned that scientists may have less than two months to settle on an updated vaccine design if they hope to have doses ready for a September rollout.

“If you’re not on your way to a clinical trial by the beginning of May, I think it’s going to be very difficult to have enough product across manufacturers to meet demand,” said Robert Johnson, deputy assistant secretary of the Biomedical Advanced Research and Development Authority, part of the U.S. Department of Health and Human Services.

The meeting was held to discuss plans for future COVID-19 vaccine booster doses and the process for selecting strains of the SARS-CoV-2 virus for vaccines to face current and future variants. 

Current COVID-19 vaccines are designed to stop the spread of the original virus that emerged in Wuhan, China, just over two years ago. Since then, the virus has evolved, and the current predominant strain—Omicron BA.2—has proved it can evade the protection conferred by these first vaccines and cause infections among the vaccinated, albeit with milder infections.

Dr. Doran Fink, a deputy director in the vaccines division of the FDA, said at the meeting that current vaccines were “not well matched” to the Omicron BA.2, though they provided some protection that was improved by boosters.

In this context, efforts have been focused on rolling out second boosters for older people and the immunocompromised. On March 29, the FDA authorized a fourth dose of the Pfizer or Moderna vaccines for those 50 and over at least four months after their previous booster.

But the FDA authorization comes at a time when the efficacy of repeated dosing of the original vaccines has come into question. Dr. Peter Marks, the head of the FDA’s Center for Biologics Evaluation and Research (CBER), called boosters a “stopgap” solution to shield the most vulnerable people from COVID while a decision was made on whether and how to update vaccines to face future variants.

“We simply can’t be boosting people as frequently as we are,” he told the VRBPAC meeting, the AP reports.

A major study of Israel’s use of a second booster jab in those 60 and older during a period when Omicron was prevalent found that while a fourth shot offered strong additional protection against severe illness for at least eight weeks, protection against infection waned by week six.

The study, published Tuesday in The New England Journal of Medicine, covered only a two-month period, so it was unclear if protection against severe illness continued beyond the eight-week cutoff. 

Next wave

The FDA’s Marks noted that heading into the fall and winter, the U.S. faced a confluence of factors that could “put us at risk for another major wave of infections”: The immunity of the U.S. population will be waning; the virus will have had six more months to evolve; and people spend more time inside during the colder season, which makes it easier for respiratory diseases to spread.

In this situation, continued boosting can be useful, but reformulating the vaccines for the winter months may be necessary.

“It seems reasonable to give a second booster to vulnerable groups. Even if the protection is short-lived, this allows countries to buy time at a moment in which all tend to lift all restrictions,” Rafael Bengoa, a former WHO director, told Fortune. “Meanwhile vaccines need to be reformulated to be effective against all coronavirus variants.”

The current average of daily COVID infections in the U.S. stands at around 30,000, down from a high of 800,000 in mid-January, according to the New York Times.