INFLUENZA VACCINATION HISTORY

The virus strains recommended for the 2015–2016 Northern Hemisphere influenza vaccines were A/California/7/2009 (H1N1pdm09)–like, A/Switzerland/9715293/2013 (H3N2)–like, B/Phuket/3073/2013-like (Yamagata lineage) viruses, and (for quadrivalent vaccines) B/Brisbane/60/2008-like virus (Victoria lineage). Study staff reviewed the electronic immunization records for all participants to determine receipt of influenza vaccinations. Electronic immunization records varied among sites and included electronic health records, employee health records, and state immunization registries. During enrollment interviews, participants (or their parents or guardians) also provided information regarding whether they received an influenza vaccination for the current season, including information on vaccine type (nasal spray vs. injection) (see the Methods section in the Supplementary Appendix). Influenza vaccination history for the current season was defined with the use of electronic immunization records and data reported by the participants, as described previously.⁸ Vaccination status for the previous season was defined with the use of electronic immunization records.