The FDA has until the early summer to decide whether vaccine makers need to change existing Covid shots to target different virus variants to avoid another possible surge in cases this fall, according to a top official at the drug regulator.

Dr. Peter Marks, who leads the FDA office responsible for vaccine safety and efficacy, told the drug regulator’s advisory committee Wednesday that a decision would need to be made by June in order to have shots available in the fall. Marks said the U.S. could face another wave of infection at that time because the virus will continue to evolve as immunity from the current vaccines wanes.

Robert Johnson, a senior official at the Biomedical Advanced Research and Development Authority, said at the meeting that the biggest challenge in updating the shots will be coordinating across the vaccine makers to make sure they are focusing on the correct Covid variants.

Pfizer, Moderna and other vaccine makers are conducting clinical trials on omicron-based shots. However, the companies aren’t currently coordinating on their new vaccine formulas, according to Jerry Weir, head of the FDA’s division of viral products. Several FDA advisory panel members said public health authorities need to develop a unified approach across vaccine makers, similar to their work updating the flu vaccine to target new strains every year.

Dr. Paul Offit, an FDA committee member, said the CDC needs to take the lead in deciding when the vaccines are no longer effective against severe illness, so the FDA and National Institutes of Health can then work with the companies to determine the best path forward on a new shot.

“At some level, the companies kind of dictate the conversation here,” Offit said. “You often hear that the company now has an omicron specific vaccine, or vaccine they can now link with the influenza vaccine. It shouldn’t come from them, it really has to come from us.”

FDA officials proposed using the process for developing new flu vaccines as a guide for changing the Covid shots. Every year, the World Health Organization makes a recommendation about the composition of the flu vaccine. The FDA than makes its own determination, based on a recommendation from its committee, about which strain should go into shots for the U.S.

The uncertainty over the trajectory of Covid’s evolution, unlike the predictability of the flu, makes it difficult to determine how the vaccines should be updated — or if they even need a change at all.

Three doses of Pfizer or Moderna’s vaccines were more than 80% effective at preventing hospitalization among healthy adults during the omicron wave, according to CDC data presented at the meeting. But the companies’ vaccines are still based on the original version of the virus that emerged in Wuhan, China, and their effectiveness against infection have substantially dropped since the beginning of the pandemic.

Covid has mutated two to 10 times faster than the flu, depending on the strain of the latter virus, according to Trevor Bedford, a virologist at the Fred Hutchinson Cancer Research Center. Bedford said he expects the spike protein, which the Covid virus uses to invade human cells, will keep evolving. The vaccines target the spike and as the protein mutates, the effectiveness of the shots can diminish.

Bedford said the most likely scenario over the next year is omicron and its subvariants will evolve to become more transmissible and further escape immunity from vaccination and infection. However, he said it’s difficult to predict whether another heavily mutated variant will come along that upends the pandemic response as omicron did over the winter.

“We really don’t know whether these wildly divergent viruses will be a common feature, or a rare feature of endemic SARS-CoV-2 evolution,” Bedford said, using the scientific name for the virus that causes Covid.

Johnson noted that in the case of the flu, the vaccine makers are able to develop production plans in advance based on a stable seasonal market. However, it’s not yet clear if Covid will follow a predictable seasonal pattern similar to the flu, according to Dr. Kanta Subbarao, a virologist who works on the flu for the World Health Organization.

FDA committee members said the central question in updating a vaccine is determining what metric public health authorities should use to determine when the shots have lost their effectiveness. Scientists have not yet determined if a certain antibody level generated by a vaccine translates to clear protection against the virus, according to Dr. Cody Meissner, an infectious disease experts at Tufts University School of Medicine.

As a consequence, public health authorities will have to rely on hospitalization rates to determine if the vaccine is losing its effectiveness, Meissner said. However, it’s also not clear if national hospitalization data is primarily made up of patients admitted due to the virus, or people who tested positive after admission for other reasons. Meissner pointed to data from Massachusetts that showed 65% of the 219 people hospitalized with Covid as of April 5 were actually admitted for reasons other than the virus.

Dr. Amanda Cohn, a CDC official, told the committee that repeated boosting is not a sustainable public health strategy. Cohn said the vaccines effectiveness against hospitalization remains high, and society may have to accept a certain level of infection which it can then treat with antiviral pills that are now on the market.

The FDA authorized fourth shots for adults ages 50 and older last week without consulting the committee, a decision that has divided scientists and physicians, some of whom believe that there isn’t sufficient data to support additional shots. Marks said the FDA didn’t consult the committee because the drug regulator viewed the authorization as a way to give people more vulnerable to severe disease additional protection until a broader decision is made for the rest of the population.

“I think we’re very much on board and with the idea that we simply can’t be boosting people as frequently as we are,” Marks told the committee. “I’m the first to acknowledge that this additional fourth booster dose that was authorized was a stopgap measure until we got things in place for the potential next booster given the emerging data,” Marks said.