Effectiveness of 1 Dose of Influenza A (H1N1) 2009 Monovalent Vaccines in Preventing Reverse-Transcription Polymerase Chain Reaction-Confirmed H1N1 Infection Among School-Aged Children in Maine

Background. In late October 2009, school-located pandemic vaccination was initiated in Maine before or concurrent with 2009 pandemic influenza A (H1N1) virus (pH1N1) peak activity. Methods. A case-control evaluation of 2009 H1N1 vaccine effectiveness was conducted in schools in Cumberland County, Maine. A case was a child who had an acute respiratory illness during 2 November-18 December 2009, and who tested positive for pH1N1 by real-time reverse-transcription polymerase chain reaction (rRT-PCR). For each case, ≥4 event time-matched controls were sampled among classmates present in school during the study period who did not have an influenza-like illness. Vaccine effectiveness was calculated as (1 - adjusted odds ratio [aOR]) × 100%; aOR was estimated by using weighted logistic regression. Results. After adjusting for a diagnosis of asthma, 1 dose of 2009 H1N1 vaccine provided 69% protection (95% confidence interval (CI), 13-89) against rRT-PCR-confirmed H1N1 infection. Vaccine effectiveness estimates for live attenuated and inactivated vaccine were 81% (95% CI, -37 to 97), and 58% (95% CI: -39 to 87), respectively. Conclusions. One dose of monovalent pandemic vaccine provided substantial protection against pH1N1 infection among school-aged children.