Flannery B, et al. - In view of the reduced effectiveness of influenza vaccine during 2015–2016 in some age groups as compared to that in previous 2009 pandemic influenza A(H1N1) virus (A[H1N1]pdm09 virus)–predominant seasons, researchers investigated if the age at first exposure to specific influenza A(H1N1) viruses could influence vaccine effectiveness (VE). Observations revealed that reduced VE against influenza A(H1N1)pdm09-related illness during 2015–2016 could be associated with early exposure to specific influenza A(H1N1) viruses.