As US officials move to restrict vaccines, including the shots to prevent respiratory syncytial virus (RSV), more evidence is emerging to confirm how dramatically the jabs reduce hospitalizations.

Announced last week as part of new restrictions on one-third of all routine childhood vaccines, RSV shots are now recommended only for high-risk babies, instead of all infants. The Trump administration announcement was led by prominent vaccine critic and health secretary Robert F Kennedy Jr.

The UK, Australia, and Denmark only recommend the RSV shot for high-risk newborns because those countries all recommend the vaccine during pregnancy. Additionally, those countries have universal healthcare – which the US does not – which means everyone is able to visit the doctor regularly and receive the vaccines. In the US, where care is much more fragmented, only about a third of pregnant people receive the shot, according to the US Centers for Disease Control and Prevention (CDC).

Infections and hospitalizations from the virus, which is particularly dangerous for young children, are beginning to ramp up in the US this respiratory season. But in recent years, doctors and parents have seen a major decrease in severe illness among babies who received the shots.

“It’s easy to see in real life. We can really tell that hospitalizations are down,” said Richard Rupp, professor in pediatrics at the University of Texas Medical Branch and the interim director of the Sealy Institute for Vaccine Sciences, who as a researcher was involved with the trials of RSV shots for newborns. “It’s made a big difference.”

Four studies in Jama confirm these observations, echoing previous data showing a significant decline in hospitalization because of the shots.

This time of year, hospitals used be full of babies struggling to breathe because of RSV; sometimes, they would run out of beds for all of them. Doctors like Rupp are now worried restricting the shots will make RSV hospitalizations surge once more.

The vast majority (81%) of babies hospitalized with RSV have no underlying conditions. Giving the shot only to children with existing health issues will “miss a large majority of the potential cases”, says Kevin Ault, an obstetrician-gynecologist and a member of the now-defunct RSV working group at the Advisory Committee on Immunization Practices (ACIP).

“That’s very concerning, and that’s why a universal recommendation was made,” he said.

Rupp called RSV “a horrible disease” and described how it can manifest in a patient: “At first, it might seem like a child has a little cold, but then things can change – and quickly. In an hour or two, it can turn the other way,” with the child “fighting for every breath … The fear is always that they’ll just stop.”

RSV vaccination during pregnancy is 70% effective at preventing hospitalization, and the shot given to newborns to prevent RSV was even better, at 81% effective, one study found. The newborn shot is more effective at preventing child hospitalization compared to the pregnancy shot, though both worked well, according to a second study.

Notably, the RSV shot for infants appears to prevent hospitalization from all lower-respiratory infections, a third study found. That’s likely because RSV can cause lingering effects, such as bacterial infections. And the vaccines are safe to give during pregnancy, a fourth study found. In the clinical trials, there was a statistically insignificant possibility of preterm birth, but this follow-up study found no such relationship.

Prior to the shots, which were approved in 2023, 2 to 3% of babies in the US were hospitalized with RSV.

“It’s the most common reason for children under five to be hospitalized,” said Ault. RSV has also been linked with developing asthma, a lifelong condition that can be dangerous.

The shots given to infants aren’t typical vaccines; they are monoclonal antibodies, or lab-made antibodies that help the babies fight off the virus for several months, until the protection fades.

Experts were surprised when the Trump administration began targeting the shots.

“I was caught, I’ve got to say, flat-footed on the whole RSV thing,” Rupp said. “I was surprised, because this sort of seems like an attack on monoclonal antibodies.”

With Covid, even people who were opposed to vaccination have been eager to get the monoclonal antibody treatments, Rupp pointed out. Earlier versions of monoclonal antibodies to fight RSV have been given to children for decades without any safety signals emerging.

“I could not even imagine that we would be at this spot with anybody even doubting that it was something that should be available for all the kids,” Rupp said.

Ault said the decision was “made by political appointees without a scientific basis”.

Tracy Beth Høeg, who was recently named the top drugs regulator at the US Food and Drug Administration (FDA), claimed recently that babies in the clinical trials had a higher death rate – but she also noted that it was not statistically significant. Even so, the FDA launched an investigation into the safety of the shots.

There were three deaths in the trials, Ault said, “but they were months and months and months after the vaccine, and they were due to things like dehydration”. The shots have since rolled out to millions of infants with no known safety concerns.

The limitations on shots will further erode access to them – even for babies who are deemed high risk, Ault said, adding: “If you start having [more] fragmentation of the system, you’re going to see hospitals and offices not stocking it. So even if it’s covered on paper, it might not be available.”

This decision means hospitals, pediatricians, and state and local governments are now scrambling, during RSV season, to figure out best practices, Ault said, which will likely result in confusion about who should stock and administer the shots. A pediatrician might assume a baby got the shot at the hospital, while hospitalists might believe the parents need to talk to their pediatrician first.

“There’s a quick breakdown in coordination and coverage that way,” Ault said.

More than half of children in the US receive their shots through the federal Vaccines for Children program. While officials have said the restricted shots will still be available through Medicaid and other federal programs, experts worry that could change with the stroke of a pen from health officials.

“Everybody’s big concern is whether or not it will be covered by insurance and by the Vaccines for Children program,” Rupp said. “Those are the people who really will not be able to afford to have this for their children.”

There’s also another complication – though it could work in patients’ favor. It’s not clear exactly which children are now recommended to get the RSV shot.

“There’s no defined ‘high risk’,” Ault said. Because of the high hospitalization rate for babies with no preexisting conditions, simply being an infant should make them eligible for the shot, Rupp and Ault both said.

“All babies in the first few months of life are at high risk for RSV,” Ault said.