Assessment of influenza vaccine effectiveness in a sentinel surveillance network 2010-13, United States.

BACKGROUND: Influenza vaccines are now widely used to reduce the burden of annual epidemics of influenza virus infections. Influenza vaccine effectiveness (VE) is monitored annually to determine VE against each season's circulating influenza strains in different groups such as children, adults and the elderly. Few prospective surveillance programs are available to evaluate influenza VE against medically attended illness for patients of all ages in the United States. METHODS: We conducted surveillance of patients with acute respiratory illnesses in 101 clinics across the US during three consecutive influenza seasons. We analyzed laboratory testing results for influenza virus, self-reported vaccine history, and patient characteristics, defining cases as patients who tested positive for influenza virus and controls as patients who tested negative for influenza virus. Comparison of influenza vaccination coverage among cases versus controls, adjusted for potential confounders, was used to estimate VE as one minus the adjusted odds ratio multiplied by 100%. RESULTS: We included 10,650 patients during three influenza seasons from August 2010 through December 2013, and estimated influenza VE in children 6m-5y of age (58%; 95% CI: 49%-66%), children 6-17y (45%; 95% CI: 34%-53%), adults 18-49y (36%; 95% CI: 24%, 46%), and adults ≥50y (34%, 95% CI: 13%, 51%). VE was higher against influenza A(H1N1) compared to A(H3N2) and B. CONCLUSIONS: Our estimates of moderate influenza VE confirm the important role of vaccination in protecting against medically attended influenza virus infection.