RESUMO
OBJECTIVE: Seasonal influenza epidemics are a major public health concern. They are caused by the influenza A and B viruses; although the A virus is more prominent, influenza B virus infection causes a disease with similar characteristics. There are two phylogenetically distinct influenza B lineages (B/Victoria and B/Yamagata), only one of which is present in the trivalent vaccine formulated each season. METHODS: Epidemiological data from the Spanish Influenza Surveillance System for 2007 to 2017 were reviewed to establish the relative proportion of each type of virus and the characterization of the B lineages in relation to the composition of the trivalent vaccine. RESULTS: The median proportion of B (2007-2017) was 27.2% (0.7%-74.8%) vs. 16.3% (0.4%-98.6%) for A-H3 and 44.2% (0.1%-98.0%) for pandemic A-H1N1 (20092017). The B lineages co-circulated in 8/10 seasons and there was mismatch with the B vaccine strain in 4/10 seasons. The B virus was dominant in 2007/08 and 2012/13 throughout Spain. There was a combination of dominance/codominance of influenza B and mismatch with the vaccine lineage in at least one third of epidemic seasons reviewed. CONCLUSIONS: Epidemiological information on influenza B has been less compiled in comparison with data on the A virus. Influenza virus type B is responsible for a significant number of cases in almost all seasons. The predominant B lineage in each season is unpredictable, affecting the protection conferred by the seasonal vaccine. Spanish epidemiological data support the rationale for a quadrivalent vaccine with both B virus lineages similarly to data from other settings.
