What is the impact of the additional B strain in FLUZONE® High-Dose Quadrivalent that is not included in FLUAD®, for adults 65 years of age and older?

Influenza is a virus that mutates (changes) from year to year. For this reason, it’s necessary to produce a new vaccine every year. Each year, World Health Organization scientists decide what the most likely influenza strains will be and determine what influenza strains to include in the vaccine for that year. Influenza vaccines are made up of two A strains of influenza and one or two B strains. 

 

There are two lineages (think ancestors) of influenza B virus: Victoria and Yamagata. The quadrivalent vaccine (FLUZONE® HIGH-DOSE QUADRIVALENT) includes both lineages of influenza B virus (Victoria and Yamagata) and two A strains. The trivalent vaccine (FLUAD®) includes only one B strain (Victoria lineage) and two A strains. 

 

The Yamagata lineage is not included in the trivalent vaccine, but this is not expected to have much, if any, impact on vaccine protection compared to the quadrivalent vaccine during the 2023-24 season. There has been very little detection of the Yamagata lineage globally since the start of the SARS-CoV-2 (COVID-19) pandemic. Instead, most of the influenza B viruses circulating have been Victoria lineage. Both the trivalent and quadrivalent influenza vaccines contain the same Victoria lineage.

 

Studies have shown the trivalent vaccine still provides some cross-protection for the other B strain even when the influenza B lineage chosen for the trivalent vaccine differs from the circulating lineage.