Introduction: There is an ongoing threat of an influenza pandemic emerging from zoonotic infections. As part of ongoing risk assessments for susceptibility to potential pandemic viruses it is useful to examine population immunity both before and after seasonal influenza vaccinations to see what the level of immunity is present and if it is boosted by vaccination with the annual influenza vaccine. There is little data on human antibody levels of Australians against local animal influenza viruses that are circulating.
Methods: Human serum panels (n=192) taken from paediatric, adults and elderly subjects prior to and following vaccination with the 2023 Southern Hemisphere influenza vaccine were tested by haemagglutination inhibition assay (HI) against recent animal and zoonotic influenza viruses isolated in Australia. Influenza viruses tested were A/Emu/Vic/23-02903/2023 (A(H10N7)) and two swine-origin viruses isolated from humans, A/South Australia/85/2018 and A/South Australia/1/2021 A(H3N2v).
Results: Increased geometric mean titres against recent circulating seasonal human influenza viruses were observed in the cohort post vaccination as expected. Responses to the animal influenza virus A/Emu/Vic/23-02903/2023 (A(H10N7)) showed no detectable antibody levels in any of the human serum panels. Serum antibody levels with a HI titre of ≥40 against A/South Australia/85/2018 A(H3N2v) were detected in 43% of pre-vaccination and 51% of post-vaccination sera with similar results seen against A/South Australia/1/2021 A(H3N2v) 34% (pre-vaccination) and 41% (post-vaccination).
Conclusion: Low seroprevalence is a risk factor for the human population against potential zoonotic influenza viruses. Vaccinating with seasonal influenza vaccine will in general not boost antibody levels against many of these viruses, especially those of avian origin. No cross-reactive antibodies were detected against the A(H10N7) virus, however good levels of cross-reacting antibody were seen against the swine A(H3N2v) viruses, and these were boosted following seasonal influenza vaccination and should offer good levels of protection against infection.