On Jun. 22, 2016, the U.S. Centers for Disease Control and Prevention’s (CDC) Immunization Practices Advisory Committee (ACIP) voted that live attenuated influenza vaccine (LAIV4) should not be used.

Vaccine virus strains included in the 2016–17 U.S. trivalent influenza vaccines included an A/California/7/2009 (H1N1)–like virus, an A/Hong Kong/4801/2014 (H3N2)–like virus, and a B/Brisbane/60/2008–like virus (Victoria lineage). Quadrivalent vaccines included an additional influenza B virus strain, a B/Phuket/3073/2013–like virus (Yamagata lineage).

Influenza A and B are the two types of influenza viruses that cause epidemic human disease. Influenza A and B viruses are further separated into subtypes (for A viruses) and lineages (for B viruses) on the basis of antigenic differences. Influenza A viruses are categorized into subtypes on the basis of characterization of two surface antigens: hemagglutinin (HA) and neuraminidase (NA). Influenza A(H1N1) viruses, influenza A(H3N2) viruses, and influenza B viruses co-circulate globally. New influenza virus variants emerge as a result of point mutations and recombination events that occur during viral replication, resulting in frequent antigenic change (i.e., antigenic drift). Immunity to surface antigens, HA and NA, reduces likelihood of infection. Antibody against one influenza virus type or subtype confers limited or no protection against another type or subtype. Frequent emergence of antigenic variants through antigenic drift is the virologic basis for seasonal epidemics and necessitates consideration for adjustment of vaccine viruses each season.