The US Food and Drug Administration (FDA) today (Aug. 21) approved the first vaccine for respiratory syncytial virus (RSV) for pregnant women, which is designed to protect newborns from the infection.
The shot becomes the second RSV vaccine, and third new intervention against the disease this year. In May, the FDA approved the Abrysvo vaccine for preventing RSV in people 60 years and older. The same vaccine is now approved for pregnant people in their third trimester. And in July, the agency approved nirsevimab, an antibody-based injection that protects babies immediately after birth from RSV.
The new interventions have been a long time coming in RSV treatment, which in the U.S. sends 58,000 to 80,000 children under age five to the hospital each year and kills 100 to 300 annually. Pfizer, which makes Abrysvo for both older people and now those who are pregnant, has been developing the shot since the 2010s, after scientists at the National Institutes of Health discovered just the right form of RSV to capture in the vaccine and present to the immune system. In a study reviewed by the FDA involving pregnant women in 18 countries, the vaccine was 81.8% effective in preventing severe respiratory illness needing medical attention in babies in the first 90 days after birth.
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Vaccinating mothers to protect infants
Preventing infection is critical to protecting the health of newborn babies, says Dr. Melissa Squires, a pediatric neuroclinical care doctor at Cincinnati Children’s Hospital who joined the study when she was pregnant with her second child in 2019. Part of the reason she enrolled to become a participant in the trial was personal experience—when her first daughter, Teresa, was three months old, she became very sick with a respiratory infection and couldn’t breathe well or eat or drink properly. “Those were a very tense few days,” she says. “I don’t know what she had because we didn’t have her tested, but she was quite sick. Anything I can do to prevent anyone going through that, I was very onboard with exploring.”
Her experience with her second daughter has been very different. Natalie was born in December 2021, during RSV and flu season, so joining the trial was “a great opportunity to prevent illness in my baby who was going to be born in the middle of respiratory disease season,” says Squires. So far, Natalie hasn't had a major infection through the first year of her life. While Squires doesn’t know if she received the vaccine or a placebo, she and her husband are happy to be spared the anxiety of worrying about a sick newborn.
Abrysvo works by co-opting the expectant mother’s immune system, prompting it to produce antibodies against RSV. Those antibodies are then passed on through the placenta to the developing fetus, so that at delivery, the baby has amassed a certain level of immune defenses that can protect it from any viruses it encounters.
“I’m a big believer in maternal vaccination,” says Dr. Elizabeth Schlaudecker, medical director of the division of infectious diseases at Cincinnati Children’s and one of the principal investigators of the study. “It’s a very safe and effective way to protect young babies when their immune systems are not working well on their own quite yet.” Schlaudecker notes there is precedent to such maternal immunization, including tetanus, diphtheria and pertussis as well as flu vaccines, all of which pregnant women get during the third trimester to protect their babies after delivery. Babies under six months old are the most vulnerable to infections, since their immune systems aren’t developed enough yet to fend off bacteria and viruses. “Even if we were to give them vaccines, a lot of them would be unable to mount an immune response because they are just too young,” says Schlaudecker.
Babies are particularly susceptible to getting severely ill from RSV, since their airways are so small, and any inflammation caused by the virus can make it difficult for them to breathe or clear fluids from their tiny lungs.
Studies from Pfizer show that newborns can be protected at least through their first RSV season; studies on how long the protection lasts are ongoing.
The upcoming fall and winter season in the northern hemisphere will be a test of sorts for a new RSV treatment strategy for infants and young children. With the approval of Abrysvo for pregnant people, and nirsevamab, which is meant for newborns in their first RSV season, doctors now have effective ways to prevent infection for the first time. They will have to figure out the best way to utilize both the maternal vaccine and the antibody treatment for infants to optimize protection from the virus. Some of that guidance will come from the Centers for Disease Control’s Advisory Committee for Immunization Practices, which will detail when doctors should use the new vaccine.
Knowing what vaccine to give
“This is a definitely an unprecedented time,” says Schlaudecker. "There are some unique challenges to figuring out which one to give and when.” She notes the maternal vaccine will fit relatively easily into the current schedule of shots that expectant mothers receive, and ideally, most pregnant moms would get the RSV vaccine in their third trimester.
If, however, the mother isn’t able to get vaccinated, or if she delivers early and before her scheduled RSV shot, then the antibody treatment could be an important back up to protect the baby. Any babies with weakened immune systems, or additional health issues such as congenital diseases that might make them more vulnerable to infections, could also get nirsevimab treatment even if they already have antibodies from their mothers, as an additional layer of protection.
The details of which babies might benefit from both having their mothers vaccinated, and nirsevimab, will become clearer once more data is collected from babies getting these therapies after a year or two. “I do expect that yes, we will continue to do research to look at how to incorporate these two products together,” says Schlaudecker.
In the meantime, Squires sees the benefit of taking advantage of the maternal vaccine for RSV. “I’m a fan of any kind of prevention for RSV,” she says. “Babies can receive vaccines after the first year of life, but even after they do, it takes a while before the body responds with those antibodies. So let mom do the work. Let mom make those antibodies and give them to the baby. That way, on day one they are ready to face that virus.”
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