Respiratory Syncytial Virus (RSV) is a common respiratory virus that circulates seasonally, typically peaking in the late fall and winter months. The infection often presents with mild, cold-like symptoms in healthy individuals, but it can lead to severe illness in certain vulnerable populations. RSV is a leading cause of hospitalization for infants under one year old, frequently causing serious lower respiratory tract diseases like bronchiolitis and pneumonia. Older adults, particularly those aged 60 and above or those with underlying heart or lung conditions, are also at high risk for severe outcomes. Recent scientific advancements have yielded several new preventative options designed to protect these two high-risk groups from severe RSV disease.
Differentiating RSV Preventative Products
The current landscape of RSV prevention includes three distinct types of products, each tailored for a specific population and utilizing a different mechanism. The first category consists of traditional vaccines designed for older adults, which work by stimulating the body’s own immune system to create protective antibodies. These include products such as Arexvy, Abrysvo, and mResvia, which are approved for use in adults starting at age 60, with some indications extending to ages 50 and 18 for high-risk individuals.
The second product type is a maternal vaccine, specifically Abrysvo, which is administered to a pregnant person to protect the infant. This approach relies on the passive transfer of antibodies, where the mother develops protective antibodies that then cross the placenta into the fetus. This provides the baby with ready-made immunity at birth, offering protection during their earliest and most vulnerable months of life.
The third preventative measure is a monoclonal antibody product, Nirsevimab (marketed as Beyfortus), which is intended for direct administration to infants. Unlike a vaccine, which trains the immune system to produce antibodies, Nirsevimab is a long-acting antibody delivered directly to the infant. It provides immediate passive immunity by introducing virus-neutralizing antibodies that directly attack the RSV fusion protein. This distinction is significant, as the monoclonal antibody offers immediate, time-limited protection.
Analyzing Efficacy and Protection Durations
Clinical trial data and real-world evidence confirm the high efficacy of these preventative options against severe RSV-related illness across all target groups. For older adults, the vaccines demonstrate significant protection against lower respiratory tract disease (LRTD). Specifically, in clinical trials, the Arexvy vaccine showed 82.6% efficacy against RSV-associated LRTD during the first season after vaccination.
Similarly, the Abrysvo vaccine for older adults demonstrated 85.7% efficacy against LRTD with three or more symptoms during the first season of follow-up. Real-world effectiveness studies further support these findings, showing that the adult vaccines were approximately 77% effective in preventing RSV-associated emergency department visits and 83% effective against hospitalizations. Protection for older adults is generally expected to last through two full RSV seasons, though some waning of effectiveness has been observed in the second year.
The maternal vaccine, Abrysvo, effectively transfers protection to the infant during their first six months of life. Clinical trials showed that maternal vaccination reduced the risk of the baby being hospitalized for RSV by 68% within three months after birth, and by 57% within six months. Furthermore, the vaccine reduced the risk of severe infant outcomes, such as the need for mechanical ventilation or intensive care unit admission, by 82% within the first three months.
For infants receiving the monoclonal antibody Nirsevimab, clinical trials demonstrated 81% efficacy against RSV-associated LRTD leading to hospitalization lasting through 150 days. The long half-life of this antibody is designed to provide protection for the entire typical five-month RSV season with a single intramuscular dose. Early real-world data also estimated Nirsevimab’s effectiveness against RSV-associated hospitalization to be as high as 90% in infants.
Safety Profiles and Reported Side Effects
The safety profiles for all three classes of RSV prevention products are generally favorable, with most reported side effects being mild and temporary. For the adult and maternal vaccines, the most common reactions include pain, redness, or swelling at the injection site, along with systemic symptoms like fatigue, headache, and muscle or joint pain. These mild effects usually resolve within a day or two following administration.
Safety monitoring has specifically looked at rare, serious adverse events. For the older adult vaccines, post-marketing surveillance has focused on a potential, though very rare, association with Guillain-Barré Syndrome (GBS), a neurological condition. The available data suggest the overall benefits of the vaccine in preventing severe RSV-related illness outweigh this minimal risk, especially for high-risk populations.
For the maternal vaccine, Abrysvo, initial concerns regarding preterm birth rates were addressed with further study. The final clinical trial data showed that the rate of preterm birth in the vaccinated group was comparable to the rate in the placebo group, indicating no increased risk. The safety profile of the infant monoclonal antibody, Nirsevimab, is also considered favorable, with the most common adverse events being minor injection site reactions, rash, and fever. Continued pharmacovigilance for all products has not identified any new safety signals or unexpected risks.
Official Recommendations for Administration
Official guidelines from the Advisory Committee on Immunization Practices (ACIP) delineate which preventative option is appropriate for each population and the optimal timing for administration. For older adults, the recommendation is a single dose of an approved RSV vaccine for all individuals aged 75 years and older. For adults aged 60 to 74 years, the decision to vaccinate should be made through shared clinical decision-making with a healthcare provider, especially for those with co-existing medical conditions that increase the risk of severe RSV disease.
The maternal vaccine (Abrysvo) is recommended for pregnant people during a specific window, generally between 32 through 36 weeks of gestation. This timing is designed to maximize the transfer of protective antibodies to the fetus just before birth, ensuring the infant has high levels of immunity during their first RSV season. This vaccination should occur during the seasonal window of September through January in the continental United States.
For infants, a single dose of the monoclonal antibody Nirsevimab is recommended for all babies born during or entering their first RSV season. Certain children up to 24 months of age who remain vulnerable to severe RSV disease, such as those with chronic lung disease or congenital heart disease, are also recommended to receive a dose for their second RSV season.
A crucial consideration is the non-overlap rule for infant protection, which simplifies the decision-making process for parents and providers. If a pregnant person receives the maternal RSV vaccine at the recommended time, the infant usually does not need the Nirsevimab injection. The choice is determined by which preventative measure is administered first.