Stratified Estimates of Influenza Vaccine Effectiveness by Prior Vaccination: Caution Required

Abstract

To the Editor—Ohmit and colleagues recently reported intriguing results from the Household Influenza Vaccine Effectiveness (HIVE) study [1]. An unexpected finding of this study was that the vaccine effectiveness (VE) estimate suggested no protection with repeated vaccination (VE, −45%; 95% confidence interval [CI], −226 to 35), whereas for participants not vaccinated in the prior season, the vaccine was protective (VE, 62%; 95% CI, 17–82).