The updated Covid vaccine boosters, a reformulated version targeting the BA.5 omicron subvariant, could be available around Labor Day. They’ll be the first Covid shots distributed without results from human trials. Does that matter?

Because the Biden administration has pushed for a fall booster campaign to begin in September, the mRNA vaccine-makers Pfizer-BioNTech and Moderna have only had time to test the reformulated shots in mice, not people. That means the Food and Drug Administration is relying on the mice trial data — plus human trial results from a similar vaccine that targets the original omicron strain, called BA.1 — to evaluate the new shots, according to a recent tweet from the FDA commissioner, Dr. Robert Califf.

That could be a potentially risky bet, experts say, if the shots don’t work as well as hoped.

Federal health officials hope that the new vaccines will provide stronger protection over the existing booster shots, which still target the original coronavirus strain. But the lack of data in humans means officials likely won’t know how much better the new shots are — if at all — until the fall booster campaign is well underway.

The FDA’s decision to move forward without data from human trials is a gamble, experts say, threatening to further lower public trust in the vaccines should the new boosters not work as intended.

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