Authorities in Florida have told residents under age 65 to avoid new Covid booster vaccines, contradicting advice from US health experts.
Governor Ron DeSantis' announcement came a day after the Centers for Disease Control recommended Americans older than six months get boosters.
Covid cases are on the rise in the US and other countries, although at levels far lower than earlier in the pandemic.
The CDC advice differs from policies in the UK and in most European countries.
That was a point repeatedly raised by Mr DeSantis, Florida Surgeon General Joseph Ladapo and three doctors who joined the video call on X, the site formerly known as Twitter, where the announcement was made.
The UK government will offer boosters to those aged over 65, younger people in clinical risk groups, frontline health workers and others who come in contact with vulnerable people.
The European Centre for Disease Prevention and Control has recommended EU countries should prioritise people aged 60 and older as well as other vulnerable groups.
"We continue to live in a world where the CDC and the [Food and Drug Administration], when it comes to Covid at least, are just beating their own path in a direction that seems inexplicable," Dr Ladapo said.
Mr DeSantis, who is also running in the Republican presidential race, was a strong supporter of Covid vaccines when they were first released, but has since repeatedly criticised vaccine manufacturers and raised safety fears - going well beyond opposition to vaccine mandates. Laws against mandates for employment or entry into businesses or public places have been passed in Florida and a number of other states.
On Wednesday, Mr DeSantis and the doctors on the call cast doubts on the safety and effectiveness of the booster shots and Covid vaccines in general.
"Once again, Florida is the first state in the nation to stand up and provide guidance based on truth, not Washington edicts," the governor said in a statement.
Florida's stance contradicts advice given by US officials and medical experts across the country.
"Covid-19 vaccines are the best way to give the body the ability to keep the virus from causing significant harm," CDC Director Mandy Cohen wrote in The New York Times. "Extensive studies and real-world experience have shown that they are safe and they work. And most Americans take them."
The boosters are tweaked versions of existing Covid vaccines which have been subjected to randomised controlled trials - the gold standard in scientific research - and real-world data from around the world.
Experts say the benefits of Covid vaccines far outweigh the risks, although the risk-benefit calculation is more finely drawn for younger, healthy people who are less likely to suffer severe Covid illness.
With reporting from BBC health disinformation reporter Rachel Schraer.