The following indices were used to describe the level of antibodies against HA of influenza virus in the human serum: geometric mean of HA antibody titer (GMT) and the protection rate, i.e., percentage of people with the level of HA antibodies ≥1:40 appearing after vaccination or previous infection by influenza virus (Brydak 2008). These results are shown in Figs. 1 and 2.

The highest level of anti-HA antibodies for hemagglutinin H1 was present in the age-groups of 15–25 (GMT = 55.7), 5–9 (GMT = 51.8), and 0–4 years (GMT = 51.5). The lowest level of these antibodies was in the age-groups of 26–44 (GMT = 35.3), 45–64 (GMT = 27.3), 10–14 (GMT = 26.2), and ≥65 years (GMT = 21.3). Concerning the anti-H3 antibodies, the highest GMT were present in the subjects aged 5–9 (GMT = 45.2) and 15–25 years (GMT = 44.2), followed closely by those aged 26–44, ≥65, and 0–4 years of age (GMT of 37.3, 32.1, and 29.4, respectively). The lowest values of anti-H3 antibodies were present in the subjects aged 45–64 (GMT = 20.8) and 10–14 years (GMT = 18.5). Concerning type B hemagglutinin, the highest values of anti-HA antibodies were present in the subjects aged 15–25 (GMT = 62.6) and ≥65 years (GMT = 60.3). In the remaining age-groups, appreciably lower values of anti-HA type B antibodies were present; the respective GMT values amounted to 47.1 in 0–4, 35.9 in 5–9, 29.2 in 10–14, 31.9 in 26–44, and 32.5 in 45–64 years of age. In the subjects of 15–25 years of age, GMT for antibodies for all types of hemagglutinins (H1, H3, and B) was greater than 40, amounting to 55.7, 44.2, and 62.6, respectively (Fig. 1).

Concerning the effectiveness of vaccination against influenza, the protection rate appeared age-dependent. For anti-HA type B antibodies, this rate achieved 76.7 % for people aged ≥65, 72.7 % for people aged 15–25, and 62.0 % for children aged 0–4 years. The lowest values of the protection rate were noted for anti-H3 HA antibodies, where it amounted to 8.7 and 6.7 % in the age-groups of 10–14 and 45–64 years. Considering the protection rate across all types of anti-HA antibodies in the context of age, it was at a close level of 31–40 % only in the group of 5–9-year olds. It should also be noted that the protection rate was the lowest in children aged 10–14 years for all types of hemagglutinin (H1, H3, and B), compared with the values obtained in the other age-groups (Fig. 2). According to the data distributed by the NIPH-NIH (2016), the percentage of the vaccinated children in the age-group of 5–14 years was 0.96 and 0.86 % in 2014 and 2015, respectively.

Comparing the protection rate during the consecutive epidemic seasons of 2012/2013, 2013/2014, and 2014/2015, attention should be raised to the level of anti-H1 HA antibodies. Influenza virus subtype A/California/7/2009(H1N1)pdm09, a component of a trivalent influenza vaccine recommended by the WHO for the northern hemisphere, including Poland, has been circulating in the population since 2009. The protection rate values obtained for this antigen during the three epidemic seasons were lower than 60 % in the successive age-groups (Table 2). The lowest value was observed for the range ≥65 years of age. That corresponds with the low 7–8 % of the people vaccinated against influenza in this age-group, belonging to a higher risk population. In the case of anti-H3 HA antibodies, the lowest titer was noted in the 45–64 age-group in the 2012/2013 epidemic season. Concerning the 26–44 age-group, comparable low values of the protection rate were obtained in the three successive seasons (Table 2), although the influenza virus strain was different in the 2014/2015 season than in the two preceding seasons (Table 3). Compared with the anti-H1 HA antibodies, protection rate for hemagglutinin type B was higher during the three epidemic seasons in all age-groups, reaching as much as 72.7 and 76.7 % for the 15–25 and ≥65 years old, respectively, in the 2014/2015 season (Table 2). Given a low percentage of the vaccinated population in Poland during the epidemic seasons compared in this study, amounting to about 1.5 % and 7–8 % for the 15–25 and ≥65 years old, respectively NIPH-NIH (2016), the results could be interpreted as a response of the immune system of the patients who had underwent infection with influenza virus.