Higher Immunological Protection of Pandemic 2009 H1N1 Influenza Live Virus Infection than Split Vaccine Against the Homologous Virus for Long Term Immunization in Ferret.

The study was to evaluate the long term immunological efficacy of pandemic 2009 H1N1 influenza live virus infection and split vaccine against the homologous virus challenge in ferrets. Antibodies in ferrets were monitored by haemagglutination inhibition (HI) assay for 200 days, the HI titers of both infected-only and vaccinated plus infected ferrets could maintain a high level for at least 182 days, without significant difference between the two infected groups. While one-dose and two-dose vaccinated ferrets could last a moderate antibody titers for 81 days, with its peak value at day 7 post immunization. After the virus challenge at day 207, the two groups of vaccinated ferrets shed virus for longer time than the two infected groups, while the latter two groups basically did not shed any virus particles. Furthermore, the vaccinated and infected ferrets which were sacrificed at day 211 exerted moderate immune protection against the challenge by alleviating clinical signs and lung lesion without obvious difference between groups. These data supported that both one-dose and two-dose vaccination of 2009 influenza A (H1N1) split vaccine conferred a moderate protection against challenge after 207 days, and there was no significant difference between the two groups. Either the infected only or vaccinated plus infected ones exerted more effective protective immune than one-dose and two-dose vaccination against the challenge, especially in preventing virus shedding, and vaccination primed before infection had no additional efficacy.