A cell culture-derived influenza vaccine provides consistent protection against infection and reduces the duration and severity of disease in infected individuals.

Background: Current knowledge of the consistency of protection induced by seasonal influenza vaccines over the duration of a full influenza season is limited, and little is known about the clinical course of disease in individuals who become infected despite vaccination.

Methods: Data from a randomized double-blind placebo-controlled clinical trial undertaken in healthy young adults in the 2008-2009 influenza season were used to investigate the weekly cumulative efficacy of a Vero cell culture-derived influenza vaccine. In addition, the duration and severity of disease in vaccine and placebo recipients with cell culture-confirmed influenza infection were compared.

Results: Vaccine efficacy against matching strains was consistently high (73%-82%) throughout the study, including the entire period of the influenza season during which influenza activity was above the epidemic threshold. Vaccine efficacy was also consistent (68%-83%) when calculated for all strains, irrespective of antigenic match. Vaccination also ameliorated disease symptoms when infection was not prevented. Bivariate analysis of duration and severity showed a significant amelioration of myalgia (P = .003), headache (P = .025), and fatigue (P = .013) in infected vaccinated subjects compared with placebo. Cough (P = .143) and oropharyngeal pain (P = .083) were also reduced in infected vaccinated subjects.

Conclusions: A Vero cell culture-derived influenza vaccine provides consistently high levels of protection against cell culture-confirmed infection by seasonal influenza virus and significantly reduces the duration and severity of disease in those individuals in which infection is not prevented.

Clinical Trials Registration: ClinicalTrials.gov NCT00566345.