A number of recent studies have found that there may be negative associations between prior influenza vaccines and subsequent influenza vaccine effectiveness (VE), depending upon the season and strain. The authors were interested in whether this was occurring over four consecutive influenza seasons (2011/2012-2014/2015) in Canada. They conducted a matched test-negative design in which laboratory-confirmed influenza cases and matched test-negative controls admitted to hospitals were compared. The patients were stratified into four groups according to their influenza vaccination history (not vaccinated current and prior season [referent], vaccinated prior season only, vaccinated current season only, and vaccinated both current and prior season). The authors found that, overall, there were mostly only non-significant associations between prior influenza vaccine and subsequent VE. Trends of non-significant decreased VE among patients repeatedly vaccinated in both the prior and current season relative to the current season only were seen in the A(H3N2) dominant seasons of 2012/2013 and 2014/2015. However, in 2011/2012, during which B viruses circulated, and in 2013/2014, when A(H1N1) circulated, being vaccinated in both seasons tended to lead to a higher VE in the current season against the dominant circulating subtype. Therefore prior vaccine impact on subsequent VE among Canadian inpatients was found to be mostly non-significant in this study. Even when the authors did observe a trend of negative impact, being repeatedly vaccinated was still more effective than not receiving the current season’s vaccine.
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