Gov. Ron DeSantis of Florida’s administration issued Covid-19 vaccine recommendations this week that directly contradicted federal officials’ guidance as his presidential campaign tries to use the resurgence of the virus to appeal to Republican voters.

With cases ticking up, the Centers for Disease Control and Prevention recommended on Tuesday that everyone six months and older who had not received a Covid-19 shot in the last two months receive a booster vaccine. The new shots, approved by the Food and Drug Administration this week, appear to be effective against a vast majority of Covid-19 variants now in circulation, according to data presented at a C.D.C. meeting on Tuesday.

Mr. DeSantis’s administration advised that Florida residents under the age of 65 skip the updated boosters.

“I will not stand by and let the F.D.A. and C.D.C. use healthy Floridians as guinea pigs for new booster shots,” Mr. DeSantis, who has a history of downplaying the efficacy of Covid-19 vaccines, said in a statement after he hosted an online panel Wednesday to discuss the new federal guidelines.

Appearing alongside Mr. DeSantis was Dr. Joseph A. Ladapo, Florida’s surgeon general, and other medical doctors who expressed skepticism about the shots.

“What I have directed our department to do is to provide guidance that really recommends and advises against the use of these mRNA Covid-19 vaccines for anyone under 65,” Dr. Ladapo said during the panel on Wednesday.

Covid-19 cases have steadily increased since July, and public health officials have warned of a comeback for the virus in the fall and winter months — though some experts say that this year is less alarming than previous years. Conspiracy theorists, right-wing influencers and politicians have seized on the moment to stoke fears that the government would again initiate widespread shutdowns or masking requirements to help prevent the spread of the disease. But federal and state officials have not suggested that those types of measures are under consideration.

In the release accompanying Mr. DeSantis’s statement, the governor’s office said that Covid-19 vaccines had “shown little to no benefit to prevent Covid-19 infection” — a claim that is directly contradicted by a host of evidence from doctors, public health officials and infectious disease experts.

The C.D.C. specifies that while older adults and “persons with weakened immune systems” are at greater risk for hospitalization and death from the disease, “healthy children and adults can still experience severe disease.” The “benefits of Covid-19 vaccination continue to outweigh any potential risks,” the agency said. Mr. DeSantis — whose presidential campaign has floundered amid money troubles and mass layoffs — has frequently appealed to Covid-19-related concerns among the Republican base, both from the governor’s office and on the campaign trail.

Mr. DeSantis has talked up his handling of the virus in the state to contrast himself with former President Donald J. Trump, the Republican front-runner in the 2024 presidential race whom he trails badly in polls, and whose administration headed the development of the coronavirus vaccines that are now very unpopular among Republican voters. Mr. DeSantis often highlights that he was one of the first governors to fully reopen his state after a pandemic lockdown.

At a public health event in Jacksonville last week that, in the absence of formal policy announcements, resembled a campaign rally, Mr. DeSantis said: “I can tell you here in Florida, we did not and we will not allow the dystopian visions of paranoid hypochondriacs to control our health policies, let alone our state.”