Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections in infants and children younger than 2 years. The most severe and life-threatening cases typically affect infants younger than 6 months. However, older toddlers, adults with chronic illnesses or multiple comorbidities, and older adults are also at an elevated risk.
Immunization Progress
Research on both active and passive RSV immunization has advanced significantly in recent years. Following the approval of the first prophylactic antibody in 2022 and two vaccines in 2023, the prevention of severe RSV illnesses has become increasingly effective.
Current strategies include maternal vaccination to protect newborns, immunization of older adults, and passive immunization of infants using the monoclonal antibody nirsevimab. In June, the FDA approved a second monoclonal antibody: clesrovimab.
Nirsevimab
Nirsevimab (Beyfortus) has been available in the European Union since November 2022. It is indicated for RSV prophylaxis in all infants during their first RSV season and in children up to 2 years of age who are at increased risk during their second season.
Nirsevimab is a human monoclonal immunoglobulin G1 (IgG1) kappa antibody produced using recombinant DNA technology. It binds to a highly conserved site on the RSV fusion (F) glycoprotein, which is present only in the prefusion form of the viral surface protein. Nirsevimab has a markedly longer serum half-life than earlier antibodies.
The drug is administered as a single intramuscular dose adjusted according to the child’s weight. Germany’s Standing Committee on Vaccination recommends that infants born between April and September receive antibodies in autumn before their first RSV season. Those born between October and March should receive the vaccine as soon as possible after birth.
Clesrovimab
Clesrovimab (Enflonsia) is a human monoclonal IgG1 kappa antibody. Unlike nirsevimab, it binds to two sites in both the pre- and post-fusion conformations of the F protein. It is administered as a single intramuscular injection and does not require weight-based dosing.
The manufacturer has applied for approval from the European Medicines Agency, and the decision is pending.
RSV Vaccines
Three RSV vaccines, each targeting a different group, are currently authorized for use in Germany.
Arexvy: A monovalent recombinant protein vaccine that includes the RSV F protein and the AS01E adjuvant. It is approved for adults aged 60 years or older.
Abrysvo: A bivalent recombinant protein vaccine containing prefusion F antigens from RSV subtypes A and B. It is authorized for use in pregnant women to protect their infants up to 6 months of age and in adults aged 60 years or older.
mResvia: Approved in mid-2024, this is the only mRNA RSV vaccine currently available. It is indicated for adults aged 60 years or older and was recently approved in the US for high-risk adults aged 18-59 years.
Real-World Data
Although clinical trials have demonstrated the strong efficacy of RSV vaccines and antibodies in preventing severe illnesses, data from real-world immunization programs are limited. A new systematic review by UK researchers addressed this gap.
Between December 2024 and February 2025, they conducted monthly searches across the Ovid, Embase, MEDLINE, and global health databases. The review included 43 studies that evaluated nirsevimab, maternal RSV vaccination, and adult RSV vaccination. The goal was to assess uptake across countries and demographic groups.
Data from more than 1.38 million individuals in Spain, France, Italy, Luxembourg, and the US were analyzed. One study combined records from Catalonia and Andorra. Most data (86%) were drawn from electronic health records and medical registries, and vaccine data were solely from the US.
Nirsevimab Uptake
In Spain, the uptake of nirsevimab during the 2023-2024 RSV season reached 90.1% (95% CI, 86.4-92.9), the highest of any country reviewed. Infants born during the RSV season and Spanish nationals had higher immunization rates.
In the US, 51.2% of eligible infants received nirsevimab in 2023-2024. Uptake was greater among preterm infants, those with at least one comorbidity, and those from Hispanic backgrounds. France recorded 76.5% coverage, with higher rates in infants younger than 3 months than in those aged 3-12 months. Luxembourg reported 83.8%, Italy 68.7%, and Catalonia-Andorra combined 60.2%.
Subgroup analyses revealed that children who experienced RSV or other acute respiratory infections were less likely to receive nirsevimab.
Maternal Vaccination
Maternal RSV vaccine coverage during pregnancy was 30.5% (95% CI, 20.6-42.6). Uptake was significantly lower among women without health insurance or with statutory coverage than among those with private insurance. Black and Hispanic women had lower rates than non-Hispanic White women.
Older Adult Vaccination
Four population-based studies assessed RSV vaccine uptake in adults aged 60 years or older, showing an average rate of 18.2% (95% CI, 10.8-28.9) in 2023-2024. Uptake was higher among adults older than 75 years, those with comorbidities, and immunocompromised individuals. As with maternal vaccination, the rates were lower among the Black and Hispanic populations.
The researchers highlighted the concerningly low uptake among pregnant women and older adults despite the availability of effective prevention tools. They called for coordinated national, clinical, and public health efforts to improve immunization rates in high-risk populations.
World Health Organization (WHO) Guidance
In May, the WHO published its first position paper on RSV immunization in infants and young children, underscoring global urgency.
RSV is the leading cause of pediatric morbidity and mortality. In 2019, an estimated 100,000 children younger than 5 years died from RSV-related lower respiratory tract infections, representing about 2% of all deaths in this age group. Approximately half of these deaths occur in infants younger than 5 months, with 97% occurring in low- and middle-income countries. Globally, RSV accounts for an estimated 3.6 million hospitalizations annually in children younger than 5 years.
The WHO recommends that all countries implement immunization programs to prevent severe RSV disease in vulnerable groups. The choice between maternal vaccination and the use of long-acting monoclonal antibodies, such as nirsevimab, should be based on local factors, including health system integration, cost, and overall feasibility.
Germany’s Robert Koch Institute provides additional guidance in its fact sheets on RSV immunization, including details on nirsevimab and adult vaccinations.
This story was translated from Univadis Germany.
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