Originally Published: 17 JUL 23 12:56 ET

Updated: 17 JUL 23 13:07 ET

By Brenda Goodman, CNN

(CNN) — This fall, parents and pediatricians will have a new option to protect babies from a lung-attacking virus that is the leading cause of hospitalization in infants under a year old in the United States every year.

The US Food and Drug Administration approved nirsevimab to protect newborns from respiratory syncytial virus, or RSV, on Monday.

Nirsevimab, which will be sold under the brand name Beyfortus, is not a vaccine. Vaccines prompt the body to make antibodies to defend against pathogens. Instead, nirsevimab is a form of passive immunity. It’s a ready-made antibody that can bind to the virus and block it from infecting healthy cells. The immune system doesn’t have to make anything.

It’s given as a single injection to an infant before RSV season, which usually peaks in the fall and winter months. The FDA approval also allows a second injection for infants up to 24 months of age who remain vulnerable through their second RSV season.

“RSV can cause serious disease in infants and some children and results in a large number of emergency department and physician office visits each year,” said Dr. John Farley, director of the Office of Infectious Diseases in the FDA’s Center for Drug Evaluation and Research. “Today’s approval addresses the great need for products to help reduce the impact of RSV disease on children, families and the health care system,” Farley said in a news release.

The group of experts that advise the Centers for Disease Control and Prevention on its vaccine recommendations will next weigh in. The Advisory Committee on Immunization Practices, or ACIP, has already issued draft recommendations and clinical considerations for nirsevimab’s use, but the group is expected to formalize those with a vote at its next meeting.

After the CDC signs off, nirsevimab will become the second antibody available to protect young children against RSV. The other, called palivizumab or Synagis, has been used only to protect the most vulnerable babies —those born prematurely who are younger than 6 months. It lasts only short time in the body, so doctors give it once a month, starting just before RSV season, until the risk has passed.

It has been a help, but it’s only partially effective. It keeps vulnerable babies out of the hospital about 50% of the time.

The antibody in nirsevimab has been stabilized so it lasts longer in the body — four to six months — and it seems to be more potent.

It’s approved for use in all infants, even healthy infants born full-term.

“This virus can affect all babies, so not just babies with certain health conditions but even healthy babies. So as a pediatrician, I’m very excited about the idea of having an immunization to prevent serious illness from RSV,” said Dr. Rachel Dawkins, medical director of the pediatric and adolescent medical clinics at Johns Hopkins All Children’s Hospital in St. Petersburg, Florida.

Nirsevimab may not be the only option for preventing the infection this fall. The FDA is weighing whether to approve Pfizer’s vaccine for pregnant women that would also protect babies. In that case, the mom makes the antibodies, which cross the placenta to safeguard the fetus and are expected to last through an infant’s first few months of life.

That vaccine would protect babies from the moment they are born, a benefit if the infection shows up out of season. Vaccines also prompt the mom’s body to make more than one kind of antibody, which would provide broader-spectrum protection.

“I would say any strategy that we can use to help prevent RSV and infants is a great one,” Dawkins said. “So if moms are able to be vaccinated to hopefully protect their newborns, that’s great. Or if the babies are able to be given this antibody, also great, and I think there is some evidence that they work together.”

In a presentation at their June meeting, the ACIP committee thinking through the potential use of the antibody said that it may make sense to give both types of protection in certain circumstances, but that most healthy babies will need only one or the other.

“Obviously, they need to consider how, how both of those preventive products for RSV infection will coexist,“ said Dr. Michael Greenberg, a pediatrician who is North American medical head of vaccines for Sanofi, the company that is marketing the antibody.

“Nirsevimab is really the first that’s going to be able to offer the protection for all infants, sort of irrespective of when they’re born because there’s the flexibility in the timing of the administration,” Greenberg said in an interview with CNN.

In the clinical trials that led to its approval, nirsevimab was about 70% effective at cutting the risk that a baby would need a doctor’s visit for RSV, and it was about 78% effective at preventing hospitalizations due to RSV compared with a placebo, according to an FDA analysis.

That means nirsevimab would prevent 1 RSV hospitalization for every 56 infants treated. So even though antibodies like this tend one to be pricey, doctors say that if it keeps babies out of the hospital, that will probably make it worth the cost.

In studies, the therapy was generally safe and well-tolerated. A few infants — less than 1% — had skin reactions after their shots, but these went away with treatment.

“We run this gauntlet every year — RSV season,” said Dr. Frank Esper, a pediatric infectious disease specialist at the Cleveland Clinic in Ohio. “We see a lot of these infants. They come in; they can’t breathe. That’s the problem. That’s what RSV does. It causes so much swelling and secretions in their breathing tubes, called the bronchioles, that they just can’t get enough oxygen.”

RSV is the leading cause of hospitalization in infants up to 1 year old. Most babies with RSV need extra oxygen; doctors help support their breathing for two or three days, and then they get better.

“Babies are small, and because babies are small, their airways are small, and so it takes just a little amount of inflammation to clog them all up, and that’s what RSV does,” Esper said.

In rare cases, RSV can be deadly. According to the CDC, it kills between 100 and 300 infants each year. And it’s not just a threat to young children; RSV sickens and kills elderly adults each year, too. Vaccines geared toward seniors have just been approved.

“So anything that we have that can fight that problem, then the better,” Esper said.

Eight years ago, Cheryl Meany enrolled in an early study testing nirsevimab when she was pregnant with twin girls. The babies were born a little early, at 33 weeks, as twins often are, and their early arrival meant they were more vulnerable to infections. She said she felt cautious about enrolling in the study, but her doctor told her he would absolutely do it if they were his own kids.

Her newborns were given injections shortly after birth, but she didn’t find out that they’d gotten the active antibodies until almost five years later.

“I kind of had an inkling,” Meany said. “They were born in December, and we live in upstate New York, where cold and flu season is rampant around here. And I’m a teacher, and my husband at the time was working in the hospital as a respiratory therapist, our older daughter was in school, so … just germs everywhere.”

She says her twins were in day care at the time, “and they never got sick.”

“This was just the best start for them and being preemies and all of the things that could have happened,” Meany said. “I really do believe that this kept them healthy in order to grow and thrive.”