Circulation of Influenza Virus in the 2015/2016 Epidemic Season in Poland: Serological Evaluation of Anti-hemagglutinin Antibodies.

The diagnostic of influenza virus infections is possible using molecular biology methods as well as the analysis of anti-hemagglutinin (anti-HA) antibodies in the blood serum. The aim of this study was to determine the level of anti-HA antibodies in 7 age groups of patients during the 2015/2016 epidemic season in Poland. A total of 1050 serum samples were tested using the hemagglutination inhibition (HAI) assay. We confirmed the presence of anti-HA antibodies for the influenza virus strains: A/California/7/2009(H1N1)pdm09, A/Switzerland/9715293/2013(H3N2), and B/Phuket/3073/2013, which were the components of the influenza vaccine for the 2015/2016 epidemic season. The level of specific anti-HA antibodies was different in each age group. The geometric mean titers were highest at age 5-9 years, where the antibody protection level reached 61.3% against B/Phuket/3073/2013 and 52.7% for A/Switzerland/9715293/2013(H3N2) antigen. The antibody level amounted to 56.7% against for antigen B at age 45-64. In the remaining age groups, the protection levels for all hemagglutinin types did not exceed 50%. These findings confirm the urgent need to increase a persistently low influenza vaccination coverage in the Polish population, which may have had its part in the noticeable increase in the confirmed cases of influenza and influenza-like virus infection during the season.