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After giving almost unanimous support to U.S. booster shot guidance a year ago, health experts are now split over when people should get them and who will actually benefit.

Bloomberg polled 13 vaccine specialists, infectious-disease doctors and public health experts on the latest Centers for Disease Control and Prevention recommendations about getting the new booster shots from Moderna and the partnership of Pfizer and BioNTech SE. Not even half of those surveyed fully endorsed the CDC's latest guidance. Some said people should consider waiting longer than the two months recommended between doses, while others thought only higher-risk people should get another shot right now.

Those ideas will likely come up in discussions between health regulators and independent advisers next month when they meet to decide whether vaccine and booster recommendations need to be changed as the pandemic evolves.

The sentiment marks a shift from the optimism around third doses last year, when cases were soaring from omicron and immunity from the first round of vaccines had waned. When Bloomberg polled 15 experts last December on boosters, 93% said everyone eligible should get one.

"The situation is becoming more like influenza in that vaccine efficacy is modest and efficacy is mainly against serious disease," said Stanley Plotkin, a vaccinologist and professor of pediatrics at the University of Pennsylvania. "However, that is not trivial as COVID can be fatal."

The survey was done before the CDC released data Friday showing just modest benefits from the new boosters in younger adults. Bivalent boosters were 73% effective at preventing COVID hospitalizations in older adults when compared with those who'd had two or more shots of the original vaccine. But in those ages 18 and older, the new shots were just 38% effective at preventing hospitalization when compared with those who'd had at least two shots.

Part of the problem is that COVID mutations are appearing so quickly that they've outrun the vaccines.

Last year, drugmakers shifted toward developing shots that are tailored to the most commonly circulating mutations of the virus. U.S. regulators endorsed the strategy, which tracks with how flu shots are developed each year. However, public health experts say there are still a lot of open questions about using multiple boosters per year to stop the pandemic.

Variant-specific boosters are "inadequate as a long-term strategy," Peter Marks, head of the Food and Drug Administration's Center for Biologics Evaluation and Research, wrote in the medical journal JAMA this month. Updating vaccines with recipes to cover new variants "is not likely to provide the depth and breadth of protection needed to interrupt viral transmission during a prolonged period."

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COVID hospitalizations and deaths are again ticking up — mostly among seniors. Some think boosters should be reserved for these higher-risk populations, similar to the approaches adopted by regulators in the U.K. and France. Health officials in Australia took a stronger stance, recommending against all booster doses for most healthy kids under 16.

As of now, the CDC is saying everyone six months and older should get the latest omicron-targeting jab because it can help prevent severe illness, hospitalization and death going into the winter months. Not many people are heeding that advice though. Despite a revamped White House booster push helped by the U.S. Men's National Soccer Team, only 14% of people aged 5 and up have gotten the booster since it came out in September, CDC data show.

The new boosters "should help in protection against severe disease and death but it may not be strong enough to prevent breakthrough infections," said Kirsten Lyke, lead researcher at the Center for Vaccine Development and Global Health at the University of Maryland School of Medicine.

The problem is, breakthrough infections can cause people to lose confidence in boosters. And the growing number of variants appears to be making breakthrough infections more likely.

Recent subvariants of omicron have supplanted the BA.4 and BA.5 mutations that the updated boosters were designed to combat. Updated boosters may not provide much help against the new versions of omicron, one preprint study published in the journal Nature Medicine showed.

When the new boosters were initially rolled out, BA.4 and BA.5 accounted for more than 95% of the country's cases. CDC data does show that boosters helped stave off symptomatic infections when BA.4 and BA.5 were dominant, with varying degrees of success based on factors like age, months since last shot, and number of doses. These days, BA.4 and BA.5 comprise a little more than 10% of the country's COVID cases.

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All of this has left experts questioning whether to recommend updated boosters to their family members or even whether to get another jab themselves.

Monica Gandhi, an infectious-disease doctor at the University of California at San Francisco told her parents to get boosted because they are 87 and 82. Her sons, who are 12 and 14, have only gotten the first two shots. Gandhi, who is in her 50s, got two boosters before the newest shots came out but doesn't plan to get an updated booster right now.

If people do plan to get boosted again, she recommends waiting at least six months between shots, because studies have shown that boosters work better when more time has elapsed, she said.

Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia, hasn't gotten the new booster either. He'd like to see the CDC publish data on how long three doses protect healthy people against hospitalization and death.

Drugmakers have said they're working on next-generation COVID shots that last longer and don't need to be tweaked whenever a new variant emerges, but it's unclear when they'll be ready.

"It is critical that we invest in newer and better vaccination strategies," said Plotkin, the University of Pennsylvania professor. "We are at the iPhone 1 of vaccines. That is not acceptable."