4.2.1. Live-attenuated virus (LAV)

The fundamental aim of vaccination is to promote protective immunity while avoiding disease from the vaccine itself. The first generation of viral vaccines was based on empirical attenuation by repeated passage in cultured cells. Several LAVs are eligible vaccines as they meet the following criteria; they elicit a strong and protective immune response with a low risk of disease from the vaccine itself. In the present regulatory environment, the use of LAVs has also been limited by safety concerns, including reversion to wild-type virulence. Because LAVs are shed from vaccines, they sometimes present a risk to unvaccinated individuals with impaired immunity. Although LAV vaccines have been developed for many RNA viruses, the mutability of these pathogens presents unique challenges for vaccine design [21].
