Effectiveness of One Dose of 2009 Monovalent H1N1 Vaccine at Preventing Hospitalization with Pandemic H1N1 Influenza In Children 7 Months to 9 Years of Age, New York City

Background: Strategies for initial vaccination against pandemic influenza strains are dependent in part on how many doses of influenza vaccine are needed to confer protection, particularly against severe complications of influenza infection. Serologic studies inform vaccination recommendations prior to vaccine licensing; post-licensure vaccine effectiveness (VE) studies can evaluate actual vaccine performance. In New York City, the availability of a highly functioning childhood immunization registry (CIR), an active vaccination campaign followed by continued 2009 influenza A(H1N1) (pH1N1) activity, and a requirement to report hospitalized influenza cases provided an opportunity to evaluate VE of an initial dose of pH1N1 monovalent vaccine in children 7 months to 9 years old.

Objectives: Determine the VE of a first dose of pH1N1 monovalent vaccine in preventing hospitalizations in children 7 months – 9 years of age.

Methods: A case-control study was done. Eligible cases were children enrolled in the CIR who were 7 months – 9 years old and hospitalized with laboratory confirmed influenza A any time December 1, 2009 – March 31, 2010. Using the CIR, ten control-children were matched to each eligible case by date of birth and zipcode of residence. Control-children given two doses of pH1N1 vaccine before their matched case was hospitalized were excluded.

Results: A total of 78 case-children and 729 date-of-birth and zipcode matched controls were studied. VE of a single vaccine dose in preventing hospitalization >14 days after vaccination was 82% (95% CI, 0–100%, p=0.04) in children 3–9 years, but zero in children 7–35 months.

Conclusions: These findings are consistent with those from seroconversion studies and have implications for setting priorities in revaccination during the 2010–2011 influenza season and for interpretation of seroconversion studies in future pandemics. Childhood immunization registries can provide a simple, rapid assessment of vaccine effectiveness to help inform vaccine policy.