The FDA approved a new preventative shot for RSV, the leading cause of infant hospitalization in the US

What to know about the new RSV antibody shot.

One way to assess a civilization, the Atlantic’s Derek Thompson wrote earlier this year, is its performance on a simple metric: How well does it keep its children alive? Compared to peer countries, the US isn’t doing so well. But on Monday, the FDA approved a new preventative treatment for respiratory syncytial virus (RSV), a disease that is the leading cause of infant hospitalization in the US and the killer of an estimated 120,000 children worldwide in 2015. That’s good news for the US’s performance on the civilization test.

RSV can cause respiratory infection, though for most healthy people, it presents as nothing more than a mild cold — the sneezing, sore throats, and stuffy noses we know all too well. But for infants with less developed airways and the elderly who generally have more vulnerable immune systems, it can cause severe inflammation and deadly infections. Nearly every child in the US will have been infected with RSV by the age of 2.

Until earlier this week, the only approved antiviral treatment for RSV was a monoclonal antibody developed in 1998 called palivizumab, with the brand name Synagis. But it’s far from perfect: The monthly treatment is expensive, and it reduces infant hospitalizations by about 58 percent — it failed to prevent hospitalization in 63 of the 150 RSV-positive children who received it in the study. It’s also only approved for certain at-risk infants.

The new treatment — another monoclonal antibody called Beyfortus developed by Sanofi and AstraZeneca — offers a number of upgrades. It’s approved for all infants up to 24 months, not just those at high risk. Its efficacy in reducing not just hospitalizations but all doctors’ visits is up to 70 percent as compared to placebo. And immunity lasts five months, enough to cover the full RSV fall season.

What to know about RSV and monoclonal antibodies

In the US, RSV puts up to 80,000 children under the age of 5 in the hospital each year, causing up to 300 deaths. It also sends an annual 160,000 adults aged 65 and older to the hospital, claiming between 6,000 and 10,000 elderly lives. For comparison, the CDC reports that the flu caused 16,000 deaths in the elderly population in the 2019–2020 season.

Monoclonal antibodies — MABs for short — are lab-made proteins designed to hunt down a particular type of antigen, or invading substance that triggers the body’s immune response. The active component in Beyfortus is nirsevimab, a MAB with a single mission: bind to a protein on the surface of RSV, preventing it from fusing with and infecting human cells. That’s similar to the MAB treatment for Covid-19, which was designed to seek out and neutralize its spike protein. The nirsevimab shot is a single dose meant to be administered from birth for children born during the fall RSV season, or just before for those born outside it.

While Beyfortus is the first FDA-approved innovation for RSV treatment in infants since 1998, this past May, the FDA approved two RSV vaccines — GlaxoSmithKline’s Arexvy and Pfizer’s Abrysvo — for adults over the age of 60.

Developing RSV vaccines has not been easy, but more are on the way

RSV has long been difficult for scientists to develop vaccines against, largely due to its shape-shifting surface proteins. The first RSV vaccine in the 1960s did more harm than good, actually causing RSV in 80 percent of the children who received it as part of a clinical trial. Two vaccinated infants, 14 and 16 months old, later died of lung issues, a tragedy that scared off further investment in RSV vaccines. Instead of developing new treatments, scientists spent the next few decades trying to figure out what went wrong.

As Vox’s Keren Landman reports, the early days of RSV vaccines could only target the proteins after they’d already fused with a human cell. By that point, the virus was already lodged in the body, rendering the treatments ineffective. But in 2013, biologist Jason McLellan discovered a way to freeze the surface proteins before they’d merged with a cell, offering a stable window for researchers to target RSV in its earlier stages. That breakthrough launched a new generation of RSV research.

In May, the FDA also approved a Pfizer shot that would be given not to infants, but to their pregnant mothers. Immunizing mothers during pregnancy confers antibodies to their newborns, offering a seamless line of defense. However, the vote was not unanimous. While all 14 FDA advisers agreed the vaccine is effective, only 10 voted that it was safe. Some held concerns over a slight bump in preterm births in vaccinated mothers compared to placebo.

Vaccines like Pfizer’s maternal shot prime the immune system to learn how to defend itself, a form of active immunity. Meanwhile, MABs like the just-approved Beyfortus are a form of passive immunity that grant protection only as long as they remain in the bloodstream, which tends to provide a shorter window of defense.

What to expect next

The new antibody treatment is expected just in time for the fall RSV season. The CDC still needs to provide recommendations on who should receive the injection, which will likely follow the FDAs approval for infants up to 24 months. Once they do, the injections will be made available to the public.

“I don’t think that the general public is either aware of RSV or realizes what a huge change this will be,” Helen Chu, an infectious disease doctor specializing in emerging respiratory diseases, told Vox’s Keren Landman last year.

In past years, 1 to 3 percent of all children under the age of 12 months in the US wound up hospitalized due to RSV. With the new MABs on offer, that number will hopefully drop this season, and with a little luck, the pipeline of new RSV protections will keep on giving.