New RSV Shot Is a Monoclonal Antibody, Not a Vaccine

Marcia Frellick

August 21, 2023

For the first time this fall, families will be offered season-long protection for infants and some children against respiratory syncytial virus (RSV).

The US Food and Drug Administration (FDA) in July approved a prevention called nirsevimab (Beyfortus, AstraZeneca/Sanofi) and it is expected to be widely rolled out in the coming weeks as the RSV season begins.

It's not a vaccine, but a monoclonal antibody used for prevention. That may cause confusion because a vaccine for RSV was approved just 3 months ago for adults aged 60 and older. And monoclonal antibodies are often used for treatment rather than prevention.

Adding to potential confusion is the fact the Centers for Disease Control and Prevention has included nirsevimab in the Vaccines for Children program, which covers the costs for uninsured kids and makes it more accessible.

Nirsevimab is approved for infants (up to 8 months old) born during or entering their first RSV season, and in children up to 2 years of age who are still vulnerable to severe RSV through their second season.

It's recommended that all infants get one injection in their first 8 months for prevention instead of the previous monthly shots used to help prevent kids at high risk from getting severe RSV.

If monoclonal antibodies can be used for preventing disease in infants, could they become a viable vaccine alternative for adults?

Specialists say no.

That's partly because of the difference in body size. Although an injection is an option for a newborn, pediatricians suggest, it would take far too much of the treatment to work as a shot for adults.

Ruth Karron, MD, an expert in pediatric infectious diseases at Johns Hopkins Medicine in Baltimore, Maryland, said that while vaccines come in small amounts and activate immune cells, monoclonal antibodies are more like a drug, with the dose based on weight.

"You'd have to give it intravenously," for larger doses she explained, which has never been studied before and would also be very expensive. "It really couldn't be an option for adults," she said.

What's the Difference Between Vaccines and Antibodies?

Monoclonal antibodies are proteins made in a lab to mimic the immune system's ability to fight pathogens such as viruses.

Karron explained that a wide variety of monoclonal antibodies have long been used to treat diseases such as cancers and autoimmune disease. In recent years, the antibodies have been used to treat COVID.

Monoclonal antibodies have also been used to treat RSV in children, but the effects don't last long — they confer passive immunity and "when it's gone, it's gone,” Karron said.

That means kids at high risk for severe RSV have had to get monthly injections.

But with nirsevimab, the mutated antibodies stay in circulation longer so they can last 5 or 6 months, enough to cover the RSV season, Karron explained. "It's highly, highly effective," she said.

Vaccines Train the Body

"The idea with vaccines is that you engage the individual's immune system. You teach it to make antibodies," Karron said. Conversely, "you give an antibody and it's good for as long as the antibody lasts. It's not teaching your body anything."

Frank Esper, MD, a pediatric infectious disease specialist at Cleveland Clinic Children's Hospital, Cleveland, Ohio, said monoclonal antibody protection for RSV is particularly welcome. "We've been trying to make an RSV vaccine since the 1960s and have done nothing but fail miserably."

"The best thing is always a vaccine," Esper said, explaining that vaccines teach the body to make its own antibodies and confer long-term protection and are "probably more efficacious than anything that's ever manmade."

"But since we've really not done very well for pediatric RSV vaccines, nirsevimab is certainly something I'm looking forward to," he said.

Fast-Acting Monoclonal Antibodies

An advantage for monoclonal antibodies is that they start working almost immediately.

Kids can get sick with RSV in the first few months of life so the speed of the monoclonal antibodies to begin protection is important, Esper said, adding that RSV "is the worst during the first year of life."

The peak age for babies getting infected enough to require hospitalization is about 2 months, he said.

By 14 months, he said, kids' immune systems and airways have matured enough "that it's not nearly as bad."

To get protection from a vaccine, he added, "usually takes 2-4 weeks from the time you get your shot to the time you see some benefit. With an antibody, you're bypassing the processing that the body has to do, and it goes straight to 'protection' mode," Esper said. "You get protected pretty much as soon as you get the antibody."

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