In 1966, the first vaccine against respiratory syncytial virus (RSV), a formalin-inactivated RSV vaccine, was developed. This vaccine was administered to US infants and children participating in four clinical trials. Vaccinated children were subsequently exposed to RSV, and those who did not have antibodies to RSV infection before vaccination experienced more frequent and more severe RSV infection, a condition known as enhanced respiratory disease. Affected infants and children presented with shortness of breath, bronchopneumonia, and fever, and approximately 80% required hospitalization. Additionally, two vaccinated children died from their enhanced disease.⁽¹⁾

Due to the vaccine failure and the harm caused to those who received this experimental vaccine, the development of further vaccines to prevent RSV infection stalled for several decades. However, in recent years, several pharmaceutical companies have employed new strategies to develop a vaccine without the risk of increasing disease.

One approach has been to develop a vaccine for pregnant women, in the hope that maternal antibodies will be transferred to the newborn, protecting it from the disease. Strategies have also included the use of new vaccine technologies, such as gene-based vaccines,^(2-3) adjuvanted subunit vaccines,^(4-5-6-7) and others.^{(8-9-10-11-12)} RSV vaccines for adults and the elderly have also been developed and undergoing clinical trials.⁽¹³⁾