Respiratory syncytial virus (RSV) is a highly contagious respiratory virus responsible for causing acute lower respiratory tract illness in infants. It is a leading cause of infant mortality, especially in low- and middle-income countries. Despite maternal antibodies, severe RSV-associated lower respiratory tract illness typically peaks in the first few months of life, during winter epidemics in temperate climates, and during rainy seasons in tropical regions.
In a recent European study published in The New England Journal of Medicine, RSV was found to be associated with approximately 50% of hospitalizations for respiratory tract illness in children under the age of one, and approximately 60% of these illnesses occurred in infants younger than three months of age. More than 80% of RSV-attributable deaths in low-income countries do not occur in hospitals.
Maternal vaccination has been effective in protecting infants against various diseases, including tetanus, pertussis, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and influenza. However, no vaccine is licensed for use in pregnancy, specifically to protect infants from RSV. The RSV prefusion F protein-based vaccine (RSVpreF), administered during the late second or third trimester of pregnancy, is effective in protecting infants from severe RSV illness during the first few months of life, which is particularly important in low- and middle-income countries where the burden of RSV-associated lower respiratory tract illness is highest.
Despite more than 50 years of development efforts, the RSV vaccine has yet to be licensed. However, the investigational bivalent RSVpreF vaccine contains stabilized preF glycoproteins from the two main cocirculating antigenic subgroups (RSV A and B). It has shown promise in phase 2b and three trials. In the phase 2b trial, the RSVpreF vaccine administered to women in the late second or third trimester of pregnancy had an acceptable safety profile. It elicited neutralizing antibody responses efficiently transferred to infants, providing evidence of the vaccine’s efficacy in preventing RSV-associated lower respiratory tract illness in infants.
A phase 3 trial showed that maternal vaccination with RSVpreF prevented medically attended severe RSV-associated lower respiratory tract illness in infants, with vaccine efficacy of 81.8% within 90 days after birth and 69.4% within 180 days after birth. The vaccine also met the efficacy criterion for hospitalization through 180 days after birth. RSV-associated lower respiratory tract illness in young infants is associated with a high burden of illness and death across countries and health systems, especially in low- and middle-income countries.
The results indicate the RSVpreF vaccine protection across the RSV illness severity spectrum. The vaccine offers the possibility of an immune response to multiple neutralizing epitopes, thus reducing the risk of immune escape observed with some monoclonal antibodies. In contrast to the Covid-19 pandemic, RSV-associated lower respiratory tract illness within 180 days after birth constituted only 22% of medically attended lower respiratory tract illnesses due to any cause in the same period.
In conclusion, RSV remains a significant public health threat, particularly for infants in low- and middle-income countries. Maternal RSVpreF vaccination is an effective strategy in protecting infants from severe RSV illness during the first few months of life, and ongoing trials are promising in this regard. Further research and development efforts are needed to successfully develop an RSV vaccine to help reduce the burden of this disease.