What to know about the new RSV vaccine for pregnant people

Editor’s note: On September 22, 2023, a U.S. Center for Disease Control advisory board recommended that an RSV vaccine, made by Pfizer, be given between the 32nd and 36th week of pregnancy. Pregnant people would get the vaccine before and during RSV season from September to January to protect babies born from October through April. The committee wrangled over how to use the vaccine instead of or in combination with a lab-made monoclonal antibody approved by the FDA in July. In most cases, the vaccine and antibody would not be used in the same infant. The antibody would be given to babies 8 months old and younger who are born to unvaccinated people or to infants born April through October.

Doctors across the United States will likely soon have two options to protect the youngest humans from RSV.

On August 22, the U.S. Food and Drug Administration approved the first RSV vaccine for pregnant people. By vaccinating individuals before they give birth — between 32 and 36 weeks of pregnancy — immune defenses sparked by the vaccine are transferred to newborns for protection during their first months of life (SN: 4/27/23).

RSV, or respiratory syncytial virus, causes respiratory tract disease and is the leading cause of hospitalization in infants in the United States. Each year, the virus puts an estimated 58,000 to 80,000 kids younger than 5 in the hospital. The vaccine joins the antibody treatment Beyfortus (nirsevimab), which was also recently approved by the FDA, as another preventive option for defending against the virus.

“It’s great that we have new tools to protect kids,” says pediatric infectious diseases physician and virologist Betsy Herold.

In a trial of approximately 7,000 pregnant people, fewer babies born to individuals given the Pfizer-made vaccine became severely ill, compared with babies whose parents received a placebo.

The results are promising. But it’s still unknown how the new vaccine and antibody drug might be used in health care clinics across the country, says Herold, of Albert Einstein College of Medicine and the Children’s Hospital at Montefiore in New York City.

While the FDA approves drugs and vaccines, the U.S. Centers for Disease Control and Prevention makes recommendations about their use. On August 3, the CDC recommended that all infants younger than 8 months get a dose of Beyfortus, and infants ages 8 months to 19 months who are at high risk for severe RSV get a dose too.

The agency will soon weigh in on whether pregnant individuals should get the new vaccine, and possibly how it might be used in conjunction with the antibody drug.

Here are two things that advisors to the CDC will likely consider when deciding whether and how to weave the vaccine into the nation’s RSV toolbox.

A potential safety concern about the new RSV vaccine still lingers.

Data from Pfizer’s trial showed some hints that there may be a risk of premature birth — babies arriving before 37 weeks of pregnancy — in people who received the vaccine. The trial, however, didn’t include enough people to show that the vaccine was behind those early births.

In 2022, pharmaceutical company GSK halted a similar RSV clinical trial after a safety review detected a higher-than-expected rate of premature birth in people given their vaccine. So that risk was top of mind during an FDA advisory committee meeting in May that discussed the Pfizer shot’s efficacy and safety. But whether the vaccines themselves are behind the risk remains unclear.

The panel of experts unanimously agreed that the Pfizer vaccine effectively protected infants from RSV during their first six months of life. But the uncertainties about preterm birth risk divided some members on whether clinical trials showed that the vaccine was safe. Ten of 14 panelists voted that the vaccine’s ability to keep kids out of the hospital outweighed a small risk of premature birth.

The Pfizer and GSK vaccines are essentially identical, says pediatric infectious diseases specialist Paul Offit, a member of the FDA’s advisory committee and director of vaccine education at the Children’s Hospital of Philadelphia. “So, one of two things is true. Either one, they both have a problem, but Pfizer just hasn’t seen it yet. Or that two, neither of them has a problem.”  

It’s possible that the vaccine sparks inflammation that raises the risk of premature birth. But some data from GSK provide reassuring clues that vaccination itself isn’t raising the risk, Offit says.

When GSK researchers combed through the data, focusing in part on people who also received other vaccines during pregnancy, they found that the risk of preterm birth was lower than expected in the control group. “The placebo itself doesn’t prevent prematurity,” Offit says. Instead, other vaccines may have provided people in the placebo group protection from other illnesses, such as flu or COVID, that can raise the risk that babies will be born early.

Larger studies could help reveal whether any link between the Pfizer vaccine and premature birth is real, Herold says.

Relying only on an RSV vaccine for protection has limits.

Vaccinating pregnant individuals at 32 to 36 weeks before birth aims to transfer protective antibodies from parent to child mainly via the placenta. But that doesn’t necessarily work for infants born too early, who are already at a higher risk for severe RSV.

Premature babies would typically arrive before most of those protective antibodies could cross from parent to placenta to child, leaving the newborns without the full brunt of defensive benefits.

Because most of the births in Pfizer’s clinical trial were full-term, scientists don’t know whether the vaccine protects this vulnerable population, Herold says. This “doesn’t mean that [the vaccine] is not good, or that it doesn’t have potential advantages. But there are caveats.”

How long vaccine protection lasts is also unclear. Data show a reduced risk of severe RSV up to six months after birth. But protection falls off over time, Herold says.

That may in part be because many antibodies from mom transferred during pregnancy don’t stick around for a long time in babies. “Certainly, most of the hospitalizations are in the younger babies, but we also see [RSV] in 8-month-olds and 9-month-olds,” when protection from mom is dwindling. Antibodies passed from mom to babies during breastfeeding may also help ensure infants have a steady supply of protection, Offit says.   

The new drug Beyfortus, an engineered antibody that is given as a shot, will be available this fall, according to the CDC. It could especially help protect babies born prematurely and those older babies that are at higher risk. But there are questions about when to use the drug over the vaccine or vice versa, Herold says. Or “is there any rationale for using both? Is that overkill?”

Based on how the antibodies interact with the virus, each approach has its benefits. Beyfortus antibodies are engineered to attach to a specific spot on the virus and activate the immune system for protection. Those antibodies stick around in babies’ bodies longer than the antibodies from vaccination that are transferred during pregnancy.

But the antibodies made in response to the vaccine glom onto many spots on the virus, Offit says. Such varied points of attack from those antibodies would be helpful if the virus acquires mutations that make it so Beyfortus no longer recognizes it. That would be much like the coronavirus did during the pandemic, rendering antibody treatments to prevent severe COVID-19 ineffective (SN: 1/11/22).

While variants are always something to worry about, Offit says, RSV doesn’t seem to mutate as quickly as the coronavirus does. Another lab-made antibody has been available since 1998 for high-risk infants without losing effectiveness against RSV variants capable of dodging the immune system. And earlier this year, the FDA approved an RSV vaccine for people older than 60, an age group at high risk for severe RSV (SN: 5/3/23). Antibodies from vaccinated people still recognized a wide variety of RSV variants, researchers report August 23 in Science Translational Medicine.

For now, Herold says, there are many unanswered questions. But the CDC’s recommendations are probably coming soon — though the timing hasn’t been announced — and could provide some clarity.