Moderna has started testing an omicron-specific booster shot in adults, the company announced Wednesday. The trial, which will include several hundred people at two dozen sites across the U.S., comes as companies and health officials grapple with a rise in cases linked to the omicron variant. Omicron has become the dominant strain in the U.S. since it first appeared in the country late last year, with the highly transmissible virus driving up cases and hospitalizations, though early data suggests it causes less severe disease.