Two for the Price of One: Can Maternal Influenza Immunization Protect Infants Against Influenza and RSV?

Globally, respiratory syncytial virus (RSV) causes a substantial burden of disease in young infants. In 2019, an estimated 33 million RSV-associated lower respiratory tract infections (LRTIs) and 3.6 million hospitalizations occurred in children under 5 years [1]. Respiratory syncytial virus disproportionately affects infants 0–6 months of age, resulting in approximately 20% of LRTI cases but almost 40% of hospitalizations. Approximately 95% of RSV infections occur in low-income countries (LICs) and middle-income countries (MICs). Thus, these young infants are at the highest risk of RSV disease, morbidity, and mortality, and they could potentially derive the most benefit from RSV-prevention strategies. Respiratory syncytial virus–preventative products, including maternal vaccination and infant monoclonal antibodies, are approximately 64%–77% effective at preventing medically attended RSV-associated disease and 70%–77% effective at preventing RSV-associated hospitalizations [2]. However, they are not widely available. As of November 2025, only 41 countries have access to these products, and 37 (90%) are high-income countries [3]. Thus, most LICs and MICs do not have access to RSV-preventative products despite these countries having the highest-risk infants. Even in some high-income countries, such as the United States, RSV-preventative product uptake remains suboptimal [3, 4], with 57% of US infants born April 2024–March 2025 protected from RSV by maternal vaccination or nirsevimab [5].