The primary objective of this study is to retrospectively characterize the safety of Flublok in adults 18 years of age and older, in comparison with egg-based trivalent or quadrivalent inactivated influenza vaccines (IIVs), using a methodological approach designed to query the database of electronic health records (EHR) maintained by Kaiser-Permanente, Northern California (KPNC), a large medical care organization (MCO).

The primary analysis is a nonrandomized, observational retrospective cohort study. Data on adults 18 years of age and older who were enrolled in the KPNC database during the risk intervals after vaccination analyzed for this study who were vaccinated with Flublok or an IIV (either IIV3 or IIV4) will be identified for analysis. The rates of occurrence of specific MAEs of interest in pre-specified risk intervals following Flublok and IIV will be compared.

Inclusion Criteria:

Data from the EHR of all adult KPNC members 18 years of age and older, who received Flublok for active immunization against influenza as part of routine medical care within the KPNC system. The comparator cohort will include data from all adults in the same age group who received any IIV (trivalent or quadrivalent) during the period of Flublok administration.

Exclusion Criteria:

Healthcare providers are expected not to administer Flublok or IIV to individuals with known contraindication to either vaccine (see product package inserts). However, there will be no exclusions other than age and KPNC membership criteria at the time of vaccination.

The study population will include adults 18 years of age and older who are vaccinated with either Flublok or IIV within the same time period, e.g. month of the year.