As advisers to the Food and Drug Administration endorsed ending one era of the Covid vaccination campaign, they also grappled with its future.An expert committee advising the Food and Drug Administration recommended on Thursday that regulators phase out the original versions of the coronavirus vaccines, allowing Americans getting their first shots to receive the updated formulations now used as boosters.As the advisers endorsed ending one era of the Covid vaccination campaign, they also grappled with its future. F.D.A. scientists on Thursday described a broad series of possible changes to the country’s vaccination program, all of them aimed at simplifying what has become an elaborate set of inoculation guidelines and defending against wintertime hospital surges.Among their proposals was adopting a streamlined dosing schedule for all Covid vaccines, one that would apply to Americans based on their age and health risks. Under that proposal, most adults and children with previous vaccines could receive a single extra Covid shot each fall, while older adults, people with weakened immune systems and unvaccinated children could be given more than one additional dose.The expert advisers did not vote on that proposal. But they offered an outline for how they wanted federal health officials to consider making the Covid vaccination program more routine, pressing them on how to time boosters for greatest protection and formulate them for the best defenses against new variants.Jerry Weir, the director of the division of viral products at the F.D.A., said, “We think that simplification of the vaccination regimen would contribute to easier vaccine deployment, better communication and improved vaccine coverage.”Committee members overwhelming endorsed that view. “We can’t keep doing what we are doing,” said Dr. Bruce Gellin, chief of global public health strategy at The Rockefeller Foundation. “This is a good path forward.”The pandemic’s threat has significantly abated since last winter. The vast majority of Americans have been infected with the virus, been vaccinated or both, federal experts said.More on the Coronavirus PandemicLong Covid: An analysis of workers’ compensation claims in New York found that 71 percent of claimants with long Covid needed continuing medical treatment or were unable to work for six months or more.Annual Boosters: The Food and Drug Administration proposed that most Americans be offered a single dose of a Covid vaccine each fall, much as they are given flu shots.A Better Covid Winter: Some of the worst days of Covid in the United States have come as winters have settled in. But a surge in hospitalizations has yet to materialize this season.New Subvariant: A highly contagious version of the Omicron variant — known officially as XBB.1.5 or by its subvariant nickname, Kraken — is quickly spreading in the United States.Yet an average of more than 560 Americans a day are still dying from Covid, the highest toll since late August, federal data shows. Many more are suffering the prolonged effects of infections. And Ruth Link-Gelles, a C.D.C. epidemiologist, warned on Thursday that those who had received the original vaccine formulation many months ago had considerably less protection against being hospitalized for Covid than people with bivalent booster doses did.Only about 40 percent of adults aged 65 and older, and only 16 percent of those 5 and older, have received the updated booster shot that was rolled out in September. Experts have said that the doses are most urgently needed by Americans at high risk of severe disease and death from Covid: older adults, people with immune deficiencies, pregnant women and those with multiple underlying conditions.At the committee meeting on Thursday, the advisers wrestled with how frequently to offer shots, what variants to defend against, and whether the vaccines were meant to stop only hospital admissions or infections altogether.Some experts said that it was too soon to say whether annual doses were needed, as they are for the flu.“I think we need to see what happens with disease burden,” said Dr. Cody Meissner, the director of pediatric infectious disease at Tufts University School of Medicine. “We may or may not need annual vaccination. It’s just awfully early, it seems to me, in the process to answer that question.”Several new studies in recent days have reported results that, while riddled with gaps in the data and other uncertainties, their authors said reinforced the benefits of the updated bivalent booster shots. Those vaccines were designed to defend against both the original version of the virus and the BA.5 Omicron subvariant that spread across the United States in the fall.One of them, published in the New England Journal of Medicine, found that the bivalent boosters generated stronger protection than the original formulations of booster shots did.The so-called bivalent booster was about 62 percent effective against severe Covid, the study found, compared with the original booster, which was 25 percent effective. (The study examined the original booster during last summer, and the updated booster during the fall.)Another study, conducted by the Centers for Disease Control and Prevention, suggested that the updated booster shot performed as well against the newest Omicron subvariant, XBB.1.5, as it did against the earlier version of Omicron, BA.5. For adults under 50, a bivalent booster was 48 percent effective against Covid infections caused by XBB.1.5, the C.D.C. found, and 52 percent effective against infections caused by BA.5.Those results could have been skewed, however, by a number of factors. The C.D.C. study, for example, could have been affected by differences in testing behavior among vaccinated and unvaccinated people, and difficulty catching all previous infections in the group. It also remains unclear long added protection against infection from a bivalent shot will last.Nonetheless, health officials cast the data as an encouraging sign that the updated boosters were holding up against the latest variants, even though they were designed to counter versions of the virus that started disappearing this winter.Officials from Moderna, a vaccine manufacturer, cited at Thursday’s meeting a third study from Kaiser Permanente of nearly half a million vaccinated individuals without immune deficiencies. Moderna said the data showed that those who had received its bivalent booster were significantly less likely to be hospitalized for Covid symptoms or to seek urgent care than were those who only received two or more doses of the original vaccine.As of November, C.D.C. officials said on Thursday, people with a bivalent booster were experiencing a 13-fold reduction in the risk of death from Covid compared with unvaccinated people, and a twofold reduction in the risk of death compared with people who were vaccinated but had not received an updated booster.Pressed by the expert committee on Thursday, F.D.A. scientists also weighed in on the contentious question of whether updated vaccines in the future should be designed in part against the original version of the virus, as the current bivalent boosters are, or be designed solely against a new version of the virus. They indicated that future vaccine formulations could be aimed exclusively at new variants, rather than the original version of the virus.F.D.A. officials told the committee that an extensive investigation has alleviated worries that the updated booster shot made by Pfizer-BioNTech could increase the risk of strokes in people aged 65 and older. One surveillance system suggested a possible heightened risk. But federal regulators said other major safety monitoring systems failed to validate that concern and there was no reason not to recommend the shot.