Virologists are trying to keep up with the faster evolution of coronaviruses.

The rapid evolution of the coronavirus into an alphabetical soup of subvariants presents a thorny challenge for health officials: they must make far-reaching policy decisions based on little certainty biological whose viral variants will be dominant this fall or winter.

The Food and Drug Administration said in late June that it would likely update coronavirus vaccines to a recall campaign in the fall targeting the highly contagious Omicron sub-variants. But the ground is shifting beneath his feet.

In just eight weeks, the sub-variant known as BA.5 went from an anomaly in numbers from cases in the United States to the dominant version of the virus in the country, now accounting for more than three-quarters of new cases. Perhaps the most transmissible subvariant to date, it is increasing positive tests, hospitalizations and intensive care admissions across the country.

It n There is no evidence that BA.5 causes more severe disease, but the latest measurements certainly shatter the myth that the virus will subside as it evolves.

"None of us have a crystal ball, and we're trying to use every last ounce of what we can from the predictive modeling and data we have to try to get ahead of a virus which was very tricky,” said Dr. Peter Marks, one of the FDA’s top vaccine regulators, after an advisory committee recommended that the agency prioritize subvariant-specific boosters. Omicron.” For something only a few nanometers in size, that's pretty darn nifty. We're trying to do our best here."

Omicron and its offshoots have been dominant for about six months now, and whatever incarnation of the virus follows is more likely be related to the Omicron family than previous versions, said Jerry Weir, an official F.D.A. regulator.

This assumption is the best calculation that can be made for the moment, according to experts outside the F.D.A. expert panel.

"Viruses like SARS-CoV-2 are constantly evolving, and it is almost certain that new new mutants will emerge within a given six-month time frame," said Jesse Bloom of the Fred Hutchinson Cancer Center in Seattle. "But as long as those mutants are descendants or close relatives of BA.2 or BA.4/BA.5, then a vaccine booster based on BA.4/BA.5, as the F.D.A. has recommended, should match them much better than the current vaccine, even if it is not a perfect match. a fourth vaccine, from Novavax, is intended for use in the United States, but its trials were conducted before the emergence of Omicron and its effectiveness against the variant may be limited. Federal regulators have not yet considered it for a booster injection.

Omicron's family tree has grown faster since the version of Omicron known as BA.1 was first detected in late November 2021. New federal estimates showed on Tuesday that BA.5 accounted for approximately 78% of new cases in the United States last week.

Emma Hodcroft, a molecule epidemiologist and researcher at the University of Bern in Switzerland, said Omicron's evolutionary pattern diverged from that of previous variants. "Delta's children weren't dominant, but Omicron's children repel their siblings, if you will," she said. "It's hin...