Antibody response to the influenza vaccine in healthcare workers.

People vaccinated against influenza develop hemagglutination inhibition (HAI) antibodies (Ab) that bind to the virus and neutralize it. Ab titer levels are variable depending on factors insufficiently studied, and tend to decrease over time. In the present study, we analyzed antibodies responses before and after influenza vaccination in nurses working in a hospital, with the aim of: - identifying seroconversion rates to trivalent influenza vaccine one month after immunization; - evaluating if, six months after vaccination, influenza HAI Ab titer fall comparing to one-month post vaccination HAI Ab titer; - studying the association between the lack of HAI Ab response (seroconversion) assessed one month after immunization and: ◦ past influenza vaccinations, ◦ baseline (before vaccination) HAI Ab titer, ◦ baseline (before vaccination) HAI Ab titer ≥ 40 (considered as a protection titer). Hemagglutination inhibition reaction was used to assess specific HAI Ab titers against influenza A(H(1)N(1)), A(H(3)N(2)) and B virus strains included in the influenza vaccine and we used progressive dilutions of two times, starting on 1:10 until 1:20.480. Seroconversion rates, one month after vaccination, were 66.7% for A(H(1)N(1)) strain, 63.2% for A(H(3)N(2)) strain and 56.3% for B strain. The most immunogenic strain used in 2007/08 influenza vaccine was A(H(1)N(1)). Seroconversion rates after one month were negatively associated with past influenza vaccinations, baseline HAI Ab titers ≥ 40 and baseline HAI Ab titers. Six months after vaccination, 50% of participants showed a drop in HAI Ab titers to an half, for each of the considered strains, but they remain high enough to protect against the disease.