Effectiveness of nonadjuvanted monovalent influenza A(H1N1)pdm09 vaccines for preventing reverse transcription polymerase chain reaction-confirmed pandemic influenza hospitalizations: case-control study of children and adults at 10 US influenza surveillance network sites.

During 2009-2010, we examined 217 patients hospitalized with laboratory-confirmed pandemic influenza in 9 Influenza Hospitalization Surveillance Network sites and 413 age- and community-matched controls and found that a single dose of monovalent nonadjuvanted influenza A(H1N1)pdm09 vaccine was 50% (95% confidence interval, 13%-71%) effective in preventing hospitalization associated with A(H1N1) pdm09 virus infection.