High level of RSV cases shows room for improvement in vaccine awareness

Cases of respiratory syncytial virus (RSV) have reached "very high" levels in about a half dozen states and "high" levels in a dozen others, according to wastewater testing data from the CDC. We're well into only the second RSV season when vaccines against RSV have been available.

That means public health authorities are still trying to raise awareness about the availability, value, effectiveness and safety of the vaccines while researchers are still collecting real-world data on their safety and effectiveness and on their uptake. A pair of recent studies offer insight into what we're learning about RSV prophylaxis specifically in young children, one of the primary at-risk populations. The studies provide supportive data journalists can include as they cover the respiratory virus season.

There are two products available to protect children against RSV, a vaccine given during pregnancy and a monoclonal antibody called nirsevimab given to infants under 8 months old or high-risk infants 8-19 months old.

Infants should only be protected by one of these methods, not both, but the CDC, the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists all recommend using one of them to protect infants against RSV. Both products use passive immunization : rather than inducing the infant to make its own antibodies (active immunization), they provide the infant with antibodies (either from the gestating parent or from the monoclonal antibody).

RSV epidemiology and nirsevimab effectiveness

The first study, published in JAMA Pediatrics in early December, reports on RSV rates from last season and nirsevimab effectiveness. It used a test-negative case control design - considered the most reliable way to assess vaccine effectiveness - in seven U.S. academic medical centers. The 28,689 children assessed included 9,536 from the 2023-24 season and 19,153 during the 2017-2020 seasons. (The 2020-21 season did not occur because of pandemic containment measures, and the next two seasons were subsequently out of sync.)

About one in four children (23%) who saw a health care provider had RSV in both the 2023-24 season and in the seasons from 2017-2020. Pediatric hospitalization rates for RSV were also similar last season to pre-pandemic seasons, about five per 1,000 children under 5 years. Maternal RSV vaccine uptake was too low to assess its effectiveness, but nirsevimab was 89% effective against RSV needing medical attention and 93% effective against RSV hospitalization.

There are two key takeaways from these findings: First, RSV season appears to be returning to its usual seasonality after the disruption of the pandemic, which means it's easier to prepare for it. Second, there's a lot of room for improvement in uptake of the maternal vaccine and probably nirsevimab considering the overall case rates and hospitalization rates weren't much different last season than in pre-pandemic ones.

Uptake of RSV prophylaxis for infants

And that brings us to the second study, a research letter published Jan. 8 in JAMA Network Open on uptake of maternal vaccination and nirsevimab. Based on electronic medical record data from 17,251 infants in the Kaiser Permanente Northern California (KPNC) system between October 2023-March 2024, 77.5% of infants were protected by either the maternal vaccine or nirsevimab.

About a third (34%) were protected by the maternal vaccine, and 41% received nirsevimab. A small proportion (2.7%) received both, and about a third of these babies were born prematurely. The study also found that younger mothers (under 25) were less likely to get maternal vaccination but more likely to use nirsevimab. Black mothers had slightly lower overall RSV protection coverage overall (70%), which matches historical trends with other maternal and pediatric vaccines.

Those are impressive numbers that don't seem to jibe with the other study's findings. But there are a couple important caveats that suggest the data can't be extrapolated to other parts of the country: it includes only insured children in the KPNC system, and KPNC is known for strong and effective promotion of vaccines to families. In fact, KPNC physicians teach vaccine acceptance workshops, and the study's senior author, Nicola Klein, has been studying maternal and child vaccination for nearly two decades in the Kaiser system.

What it all means

Taken together, the studies suggest that high levels of RSV vaccination are possible but likely aren't occurring throughout much of the country. Without data on KPNC's case rates, it's hard to tell what impact the maternal vaccine and nirsevimab would have on RSV infections at scale. So far, the pair of prophylaxis aren't making a dent in national rates. Increased awareness of the vaccines and their effectiveness might change that.

Background primer on RSV protection in infants

The maternal RSVPreF vaccine (Abryso, made by Pfizer), approved in August 2023, is recommended to be administered between 32-36 weeks of pregnancy so that the antibodies pass through the placenta to the fetus and provide the newborn with antibodies against RSV from birth.

Based on clinical trial data, the vaccine is approximately 82-91% effective against severe RSV in the first three months after birth and 69-77% effective through six months (the range depends on when the vaccine is given during pregnancy, with the higher effectiveness after 32 weeks). This is passive immunization because the antibodies are transferred to the fetus during pregnancy rather than the fetus producing its own antibodies.

Nirsevimab is not a vaccine since it does not induce the body to create its own antibodies, but it's often referred to informally as one. It's a monoclonal antibody that also provides passive immunization by giving the infant preformed antibodies against RSV. It's recommended starting in October of each year for infants under 8 months old in their first RSV season who do not have confirmed protection from the maternal vaccine. It's also recommended for those 8-19 months old at increased risk of severe RSV disease, including immune-compromised children, those with chronic lung disease, and American Indian and Alaska Native children.

In clinical trials, it was approximately 80% effective against RSV for 5 months, which is about the length of a typical RSV season. Data from the 2023-2024 RSV season from electronic medical records' analysis showed nirsevimab to be 77% effective against RSV-related emergency room visits and 98% effective against RSV-related hospitalization.

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