Omicron-based covid booster shots will roll out this fall

The change shows the FDA trying to be more nimble in efforts to keep up with a changing virus. The precise formula has not been tested in people yet, but studies showed that vaccines tuned to fight a previous version of omicron modestly increased the short-term immune response in people compared with more shots of the original. The agency will depend in part on that data as it reviews the new vaccines.

FDA advised companies Thursday to create a two-part vaccine for a fall booster campaign. One part of the vaccine will be the original formula, based on the version of the virus that spread globally in early 2020. The other part will be based on the BA.4 and BA.5 omicron subvariants that currently make up half of the strains that are sequenced in the United States.

It is quite possible that BA.4 and BA.5 will be eclipsed by new variants by the fall, but the hope is that a new shot will help broaden immunity, since they are closer to where the virus is today. A scientist from Pfizer showed data to FDA advisers Tuesday that in mice, a vaccine based on those versions of omicron appeared to generate a stronger immune response.

Tracking the coronavirus vaccine

For a year and a half, coronavirus vaccines based on the original version of the virus have provided robust protection, particularly against severe illness. But immunity tapers off over time, and the virus has proved wily, spawning a growing Greek alphabet of new variants that are more contagious and deft at slipping by people’s immune defenses.

“As we move into the fall and winter, it is critical that we have safe and effective vaccine boosters that can provide protection against circulating and emerging variants to prevent the most severe consequences of covid-19,” the FDA statement said.

Experts have felt anguished that such a consequential decision has to be made based on very limited data. It is possible that the change could provide a detectable increase in people’s protection against severe illness and perhaps also infection, but it is not certain.

“I think the FDA here is making a best guess as to what they think is the right thing to do, and that may turn out to be a good one and it might not,” said John Moore, a professor of microbiology and immunology at Weill Cornell Medicine. “We don’t know and have no real way of knowing.”

The modified vaccine will be used as a booster. People who are still getting their first shots will continue to get the original version of the vaccine.

People who are unvaccinated or unboosted should not put off vaccination in hopes of getting a new shot, particularly given the high levels of transmission, said an FDA official who requested anonymity because they were not authorized to speak. People will still…