There are three types of influenza viruses: A, B, and C. Types A and B cause seasonal epidemics of influenza; infections from type C cause mild respiratory illnesses. Influenza A can be further categorized based on two types of surface proteins: hemagglutinin (H) and neuraminidase (N); the strains are then organized by the combination of proteins, e.g. H1N1. Influenza B is not divided into subtypes. Influenza A (H1N1), A (H3N2), and a few types of influenza B are included in each year’s vaccine, which can be either trivalent or quadrivalent. Each year small changes occur in the virus during replication producing viruses that are closely related but different, which is why yearly vaccinations are needed and the vaccination is more protective some years than others.
This year the vaccine is expected to be more protective than it was last year. This year’s vaccine is made to protect against the following viruses :
- An A/California/7/2009 (H1N1)pdm09-like virus
- An A/Switzerland/9715293/2013(H3N2)-like virus
- A B/Phuket/3073/2013-like virus (B/Yamagata lineage)
- The quadrivalent also protects against B/Brisbane/60/2008-like virus
The composition of the vaccine this year represents a change in the strains of influenza A (H3) and influenza B (Yamagata lineage) from last year . A large majority of viruses that have been sequenced so far are similar to viruses included in this year’s vaccine .
- Appiah GD, et al. Influenza Activity-United States, 2014-15 Season and Composition of the 2015-16 Influenza Vaccine. MMWR Morb Mortal Wkly Rep 2015 June 5;64(21):593-90. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6421a5.htm