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New Monoclonal Antibody and Vaccine Can Protect Kids from RSV

BY KATHY KATELLA September 25, 2023

Two new immunizations may help block a virus that overwhelmed children’s hospitals last year.

[Originally published: August 14, 2023. Updated: Sept. 25, 2023]

Last fall and winter were worrisome for many parents as children’s hospitals across the country filled to capacity with infants and toddlers infected with respiratory syncytial virus (RSV). The virus causes a runny nose and other cold-like symptoms in older children and most adults, but it can lead to severe illness in the youngest and oldest patients. A dip in cases during the COVID-19 pandemic led to lower immunity to RSV, and the disease came roaring back last year.

The upcoming RSV season could be much easier, thanks to approval from the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) this year of two new tools designed to protect vulnerable infants from the disease. The first is a monoclonal antibody called nirsevimab (brand name: Beyfortus™) that targets all infants up to 8 months old, born during—or entering—their first RSV season, and for a small group during their second season if they are between 8 and 19 months old and at high risk for severe disease (including children who are severely immunocompromised).

The second is a vaccine called Abrysvo™ to be given during the RSV season to pregnant individuals so they can pass along protection to the fetus and prevent severe RSV disease in newborn infants.

“This is extremely exciting for both parents and pediatricians,” says Thomas Murray, MD, PhD. “Nirsevimab should reduce hospitalizations. Even if it doesn't completely prevent infection, we expect it will reduce disease severity significantly.”

The CDC approvals of nirsevimab in July and the vaccine for pregnant women in September came weeks after the CDC finalized two new RSV vaccines for adults ages 60 and older. Experts hope these immunizations will head off a spike in seasonal hospitalizations for older adults, another group at increased risk for the virus.

It’s important to note that nirsevimab for children works differently than the RSV vaccines for adults (more on that below). Dr. Murray answered questions about the new preventive options for children.

1. What is nirsevimab, and how does it work?

Nirsevimab is a monoclonal antibody (designed by pharmaceutical companies Sanofi and AstraZeneca), which was given an FDA Fast Track designation, a process designed to expedite drugs to treat serious conditions and fill unmet medical needs. It’s a preventive option that mimics the immune system’s ability to fight off invaders, such as viruses, providing an extra layer of defense against RSV, according to the CDC. (It is not intended to treat patients already sick with the virus.)

As a monoclonal antibody, nirsevimab approaches prevention differently than a vaccine would. “A vaccine causes your body to produce antibodies to protect you against whatever virus the vaccine is targeting,” Dr. Murray says. “The monoclonal antibody bypasses that step.” Once introduced into the body, nirsevimab functions as a form of passive immunity that is ready to bind to the RSV virus and block it from infecting healthy cells, preventing severe disease.

2. Why is RSV protection so important for young children?

RSV is a highly contagious virus that spreads rapidly among children. The virus can live for hours on surfaces, such as doorknobs, countertops, and crib rails, and then spread when people touch their face after touching those surfaces. It also spreads through the air, often when a person infected with the virus coughs or sneezes, and droplets get in another person’s eyes, nose, or mouth. So, it's important to avoid close contact with an infected person.

The infection can make its way down into the lungs, causing a clinical syndrome called bronchiolitis (inflammation of the small airways in the lungs) that results in difficulty breathing; it’s especially risky for those born prematurely or with underlying medical conditions, such as congenital heart disease.

In children younger than age 5, there are approximately 2.1 million RSV-related outpatient visits a year in the U.S. Many babies won’t need to go to the hospital, but in a given year, 58,000 to 80,000 of them do, and there are 100 to 300 deaths.

“Young babies are nose-breathers, so if their nose gets clogged, that can contribute to respiratory problems,” Dr. Murray says, adding that RSV is also the most common cause of pneumonia. “They can develop life-threatening complications. Plus, early infection with RSV is a risk factor for developing asthma later in childhood.”

There is no treatment for RSV beyond hydration, over-the-counter medicines to control fever, if present, and close monitoring for difficulty breathing. Severely ill babies may need to go to the hospital for intravenous fluids, supplemental oxygen therapy, and, in rare cases, mechanical ventilation (a machine to help them breathe).

3. How is nirsevimab administered?

Nirsevimab is given by intramuscular injection into the thigh muscle. A single dose may cover an entire RSV season, which starts in the fall, peaks in the winter in most parts of the U.S., and can last through spring. (The cycle was disrupted during the COVID-19 pandemic, and experts hope to see it return to normal this fall.)

4. How well does nirsevimab work?

Sanofi and AstraZeneca tested the shot in 3,200 infants in studies before submitting data to the FDA. One study, published in the New England Journal of Medicine, found nirsevimab to be 79% effective against RSV severe enough to warrant medical attention. A panel of independent advisors to the FDA voted unanimously to recommend approving the preventive option.

5. Are there side effects from nirsevimab?

Common side effects are reactions, such as rashes or visible irritation, at the injection site. Nirsevimab comes with warnings about anaphylaxis (a life-threatening allergic reaction), and it should be given with caution to infants and children with clinically significant bleeding disorders, according to the FDA.

6. Why aren’t older children eligible for nirsevimab?

Older children aren’t at high risk for severe disease from RSV. Almost all children older than age 2 have had the virus already and have some immunity to protect them, Dr. Murray explains. By the time they are teenagers, they usually have strong immunity from multiple exposures and experience RSV as a mild cold. Rarely, they develop respiratory complications or severe disease.

Immunity from RSV continues well into adulthood. However, older adults start to lose immunity as they age—they're unable to fight off infections, such as RSV, as well as they did when they were younger. “That's why there are vaccines for people over 60,” Dr. Murray says.

7. What do we know about the RSV vaccine that could protect newborns?

Abrysvo, developed by Pfizer, is approved for injection into the muscle to give pregnant individuals antibodies against RSV that they would pass along to the fetus—their newborn baby would be protected for the first six months of life. The vaccine for pregnant women is administered seasonally, from September to January to provide protection of newborns when RSV infection is expected to be at its highest, between October and March.

The shot is not recommended for this use from February through August. During those months, parents of small children should talk to their pediatricians about nirsevimab, the CDC says. Most infants will need protection from one of the new RSV immunizations, but not both, it adds. An exception might a baby born less than two weeks after maternal immunization; in that case, a doctor may recommend that the baby also receive the monoclonal antibody.

Abrysvo has been shown to reduce the risk of RSV hospitalization for babies by 57% in the first six months after birth, according the CDC. .

When an independent advisory committee to the FDA voted in May to recommend approval of the shot, a few independent advisors expressed concern about a slight increase in preterm births among women who were given the vaccine—5.6% in vaccinated women compared to 4.7% in an unvaccinated group. While the 14-person panel recommended the Pfizer shot unanimously based on its efficacy, it voted 10-4 based on whether the data supported its safety. But, according to the FDA, the available data was insufficient to establish or exclude a causal relationship between preterm birth and Abrysvo. The agency is requiring Pfizer to conduct post-marketing studies to assess the signal of serious risk of preterm birth.

8. What can parents do to protect their babies from severe RSV?

Parents should help children at high risk for severe RSV take precautions, including avoiding close contact with sick people. “Parents also can teach children to wash their hands with soap and water for 20 seconds to help ensure they are not touching their faces with dirty hands after touching shared toys and other potentially contaminated surfaces,” says Dr. Murray.

Pregnant women can talk to their doctors and parents to their pediatricians about whether the immunizations are appropriate for their family, Dr. Murray adds.

“Any therapy that we can give safely and that will reduce hospitalization for babies is helpful to everybody,” Dr. Murray says, adding that fewer RSV-related emergency room visits will benefit everyone, from the infants themselves to older children and adults seeking urgent care for other ailments, who will then be able to receive prompt care in hospitals that aren’t overwhelmed.