Advisers to the FDA recently met to discuss the future of respiratory syncytial virus (RSV) vaccines for children. This followed Moderna’s forced halt of its mRNA RSV vaccine trials after alarming data showed higher rates of severe RSV in vaccinated infants compared to those given a placebo. Clinical trial data revealed 12.5% of vaccinated children developed severe RSV disease, compared to just 5% in the placebo group.

These outcomes raised alarms due to past experiences with RSV vaccines. In the 1960s, trials of a formalin-inactivated RSV vaccine led to vaccine-associated enhanced respiratory disease (VAERD), where vaccination worsened illness instead of preventing it. That trial resulted in two toddler deaths and hospitalization for 80% of the vaccinated participants. Despite decades of research, the risks tied to VAERD remain unresolved.