Influenza A(H1N1)pdm09 but not A(H3N2) virus infection induces durable sero-protection: results from the Ha Nam Cohort.
Hoa LNM., Sullivan SG., Mai LQ., Khvorov A., Phuong HVM., Hang NLK., Thai PQ., Thanh LT., Carolan L., Anh DD., Duong TN., Bryant JE., van Doorn HR., Wertheim HFL., Horby P., Fox A.
BACKGROUND:The extent to which influenza recurrence depends upon waning immunity from prior-infection is undefined. We used antibody titres of Ha-Nam cohort participants to estimate protection curves and decay trajectories. METHODS:270 households participated in influenza-like-illness surveillance and provided blood at intervals spanning RTPCR-confirmed transmission. Sera were tested in haemagglutination inhibition assay. Infection was defined as RTPCR+ influenza-like-illness and/or seroconversion. Median protective titres were estimated using scaled-logistic-regression to model pre-transmission titre against infection status in that season, limiting analysis to households with infection(s). Titres were modelled against month since infection using mixed-effects linear regression to estimate decay and when titres fell below protection-thresholds. RESULTS:295 and 314 participants were infected with H1N1pdm09-like and A/Perth/16/09-like (H3N2Pe09) viruses, respectively between December 2008-2012. The proportion of householders not-infected (protected) rose more steeply with titre for H1N1pdm09 than for H3N2Pe09, and estimated 50% protection titres were 19.6 and 37.3, respectively. Post-infection titres started higher against H3N2Pe09 but decayed more steeply than against H1N1pdm09. Sero-protection was estimated to be sustained against H1N1pdm09 but to wane by 8-months for H3N2Pe09. CONCLUSIONS:Estimates indicate that infection induces durable sero-protection against H1N1pdm09 but not H3N2Pe09, which could in part account for the younger age of A(H1N1) versus A(H3N2) cases.