Antibody titres elicited by the 2018 seasonal inactivated influenza vaccine decline by 3 months post‐vaccination but persist for at least 6 months

BACKGROUND: In Australia, seasonal inactivated influenza vaccine is typically offered in April. However, the onset, peak and end of a typical influenza season vary, and optimal timing for vaccination remains unclear. Here, we investigated vaccine‐induced antibody response kinetics over 6 months in different age groups. METHODS: We conducted a prospective serosurvey among 71 adults aged 18–50 years, 15 community‐dwelling (‘healthy’) and 16 aged‐care facility resident (‘frail’) older adults aged ≥65 years who received the 2018 southern hemisphere vaccines. Sera were collected at baseline, and 1, 2, 4, and 6 months post‐vaccination. Antibody titres were measured by haemagglutination inhibition or microneutralisation assays. Geometric mean titres were estimated using random effects regression modelling and superimposed on 2014–2018 influenza season epidemic curves. RESULTS: Antibody titres peaked 1.2–1.3 months post‐vaccination for all viruses, declined by 3 months post‐vaccination but, notably, persisted above baseline after 6 months in all age groups by 1.3‐ to 1.5‐fold against A(H1N1)pdm09, 1.7‐ to 2‐fold against A(H3N2), 1.7‐ to 2.1‐fold against B/Yamagata and 1.8‐fold against B/Victoria. Antibody kinetics were similar among different age groups. Antibody responses were poor against cell‐culture grown compared to egg‐grown viruses. CONCLUSIONS: These results suggest subtype‐specific antibody‐mediated protection persists for at least 6 months, which corresponds to the duration of a typical influenza season.