Development and Characterization of Candidate Vaccine Viruses Against High Pathogenicity Avian Influenza A(H5) Viruses for Rapid Pandemic Response.

High pathogenicity avian influenza A(H5) viruses pose a pandemic threat. These viruses have rapidly evolved in birds and frequently crossed species barriers, resulting in over 1000 confirmed human infections, with a case fatality proportion of approximately 50%. In response, the US Centers for Disease Control and Prevention (CDC) has developed dozens of A(H5) candidate vaccine viruses (CVVs) over the past 2 decades, primarily targeting clades known to infect humans. This report summarizes the development and characterization of the CVVs, with a particular focus on their antigenic relationships with clades 2.3.2.1e and 2.3.4.4b A(H5N1) viruses, which have been responsible for the majority of recent human infections.