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The inactivated quadrivalent cell-culture–derived influenza vaccine (IIV4c) was previously shown to be safe and immunogenic in children. Now, to evaluate the vaccine's efficacy, investigators randomized children (age range, 2 to <18 years) to receive IIV4c or meningococcal vaccine (control) during the three influenza seasons spanning 2017–2019 in eight countries in Europe, Asia, and Oceania. First occurrence of laboratory-confirmed influenza was assessed with active surveillance from 14 days after vaccination until the end of each influenza season.
Of 4514 children enrolled, 4509 were included in the efficacy analysis (66% were previously vaccinated, 48% female, and 51% aged 2 to <9 years). In all, 175 influenza cases occurred in the IIV4c g…