Amid a sputtering vaccine rollout and fears of a new and potentially more transmissible variant of the coronavirus, Britain has quietly updated its vaccination playbook to allow for a mix-and-match vaccine regimen. If a second dose of the vaccine a patient originally received isn’t available, or if the manufacturer of the first shot isn’t known, another vaccine may be substituted, health officials said.

The new guidance contradicts guidelines in the United States, where the Centers for Disease Control and Prevention has noted that the authorized Covid-19 vaccines “are not interchangeable,” and that “the safety and efficacy of a mixed-product series have not been evaluated. Both doses of the series should be completed with the same product.”

Some scientists say Britain is gambling with its new guidance. “There are no data on this idea whatsoever,” said John Moore, a vaccine expert at Cornell University. Officials in Britain “seem to have abandoned science completely now and are just trying to guess their way out of a mess.”