Previous studies have found that the effectiveness of influenza vaccine during the 2015-2016 season was reduced in some age groups compared to in previous influenza A(H1N1)pdm09 virus-predominant seasons. The authors were interested in whether the age of first exposure to specific influenza A(H1N1) viruses can influence vaccine effectiveness (VE). They estimated the effectiveness of the influenza vaccine against PCR-confirmed influenza A(H1N1)pdm09-associated medically attended illnesses from the 2010-2011 season through to the 2015-2016 season. A total of 2115 A(H1N1)pdm09 virus-positive and 14,696 influenza virus-negative patients aged ≥6 months were included in the study. VE was 61% (95%CI 56-66%) against A(H1N1)pdm09-associated illness during the 2010-2011 through to 2013-2014 seasons, compared with 47% (95%CI 36-56%) during 2015-2016. In particular, during the 2015-2016 season, A(H1N1)pdm09-specific VE was 22% (95%CI -7% to 43%) among adults born between 1958-1979 versus 61% (95%CI 54-66%) for all other birth cohorts combined. Therefore there does appear to be an association between reduced VE against influenza A(H1N1)pdm09-related illness during 2015-2016 and early exposure to specific influenza A(H1N1) viruses.
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