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This year's headlines on respiratory syncytial virus (RSV) focused largely on the relatively new vaccines -- including a new option, expanded FDA approvals, and updated CDC recommendations -- plus a potential new monoclonal antibody for protecting infants.

FDA Approvals and Expansions

In May, the FDA approved the mRNA-1345 RSV vaccine (mResvia) from Moderna for people ages 60 and older to protect against lower respiratory tract disease. Approval was based on results from the phase III ConquerRSV trial, which demonstrated a vaccine efficacy of 83.7% in preventing RSV-associated lower respiratory tract disease with at least two signs or symptoms and an efficacy of 82.4% against lower respiratory tract disease with at least three signs or symptoms.

In June, the FDA expanded the approval of GSK's RSV vaccine (Arexvy) to include adults ages 50-59 at increased risk of lower respiratory tract disease due to underlying chronic conditions. Previously, the adjuvanted RSV prefusion F protein-based vaccine had been approved only for adults ages 60 and older.

Then in October, the FDA expanded the approval of Pfizer's bivalent RSV prefusion F vaccine (Abrysvo) to include adults ages 18-59 with comorbidities that put them at high risk for RSV-related lower respiratory tract disease. The vaccine had previously been approved for the prevention of RSV-associated lower respiratory tract disease in adults ages 60 and older and for pregnant individuals to protect infants at birth.

Changes to RSV Vaccine Recommendations

In June, the CDC Advisory Committee on Immunization Practices (ACIP) altered their previous recommendations for adult RSV vaccination.

ACIP's recommendations now say that all adults ages 75 and older should receive a single dose of RSV vaccine. Adults ages 60-74 at increased risk of severe RSV disease may also receive a vaccine, the committee decided.

ACIP did not vote on recommending the vaccines for younger at-risk groups.

Maternal RSV Vaccine Safety

New evidence emerged on the effectiveness and safety of maternal RSV vaccines and infant immunization with the monoclonal antibody nirsevimab (Beyfortus).

A real-world study found that administration of Pfizer's maternal RSV vaccine didn't increase the risk of preterm birth or other negative neonatal outcomes. However, the vaccine was associated with modest risk of maternal hypertensive disorders.

In that study, preterm birth (i.e., before 37 weeks' gestation) was actually numerically less common for women who received the maternal RSV vaccine (5.9% vs 6.7% without it; adjusted OR 0.87, 95% CI 0.62-1.20). No significant differences emerged by vaccination status in small-for-gestational-age birthweight, stillbirth, neonatal intensive care unit (ICU) admissions, and respiratory distress with NICU admissions, or in neonatal jaundice, hypoglycemia, or sepsis. Percentages of spontaneous and non-spontaneous premature births were similar as well.

In 2022, GSK abandoned development of a maternal RSV vaccine due to an increased risk for preterm birth. FDA recommends vaccination with Pfizer's vaccine at 32-36 weeks to reduce the risk.

Nirsevimab and Alternatives

The real-world ENVIE study found that the monoclonal antibody nirsevimab protected young infants against hospitalization for RSV, pediatric ICU admissions, and mechanical ventilation. In a logistic regression model, nirsevimab was estimated to be 83% effective in preventing hospitalization from RSV bronchiolitis (95% CI 73.4-89.2) in infants younger than 12 months of age.

In another study, the CDC found that nirsevimab was 90% effective in preventing RSV hospitalizations in infants. However, the study timeframe only covered part of a typical RSV season and effectiveness over a full RSV season was expected to be lower, the researchers cautioned.

Evidence emerged that the investigational monoclonal antibody clesrovimab was effective in protecting against RSV-related disease and prevented RSV hospitalizations among healthy preterm and term infants. Compared with placebo, a single intramuscular dose of clesrovimab reduced the incidence of RSV-associated medically attended lower respiratory infections that had at least one indicator of lower respiratory infection or severity by 60.4% (95% CI 44.1-71.9, P<0.001) through 5 months. Clesrovimab also reduced RSV-associated hospitalizations by 84.2% (95% CI 66.6-92.6, P<0.001). Adverse events and deaths were similar between the clesrovimab and placebo groups.

Guillain-Barre Syndrome and RSV Vaccines

Reports of Guillain-Barré syndrome (GBS) following RSV vaccination in adults ages 60 and up were rare but still higher than expected. Data from the Vaccine Adverse Event Reporting System (VAERS) revealed that there were 1.5 reports of GBS per 1 million doses administered of the GSK RSV vaccine, and five reports per 1 million doses administered of the Pfizer RSV vaccine.

Of a total of 28 cases of GBS in older adults after RSV vaccination that were reported to VAERS from May 2023 to mid-April 2024, 60.3% occurred after receipt of the Pfizer RSV vaccine and 39.3% after the GSK vaccine.

There were two fatalities associated with GBS out of a total of 18 reported deaths from a variety of reasons. To put these numbers into context, at least 10.6 million adults ages 60 and older received an RSV shot from August 2023 (when the newly approved GSK and Pfizer vaccines were being rolled out) to March 2024.

During prelicensure clinical trials, GBS was identified as a potential safety concern, and the rates of GBS after RSV vaccination are in general alignment with what was observed in clinical trials of the vaccines, CDC researchers said.

According to the CDC, the benefits of the vaccine continue to outweigh this small risk.

Other RSV-Related News in 2024:

Cardiac Events Common in Older, Hospitalized RSV Patients

Giving RSV and Flu Vaccines Together Led to Adequate Immune Responses

Adult RSV Shots Given to More Than 30 Babies by Mistake

Most Expectant Moms Say They're Likely to Get Maternal RSV Vaccine

Jennifer Henderson also contributed to this report.

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