Skip to Main Content

Novel H1N1 Influenza Vaccine Effectiveness Study


The Novel Influenza H1N1 Vaccine Effectiveness evaluation sought to measure the effectiveness of the monovalent novel H1N1 (nH1N1) influenza vaccine in preventing hospitalization among all persons recommended to receive this vaccine.

Goals & Objectives

  • To evaluate the overall effectiveness of the monovalent nH1N1 vaccine among persons >6 months of age, in preventing hospitalization with a RT-PCR-confirmed, community-acquired nH1N1 influenza virus infection in participating EIP sites.
  • To make age-specific estimates of vaccine effectiveness for the monovalent nH1N1 vaccine in preventing nH1N1 hospitalizations for the following age groups: 7-59 months, 5-17 years, 18-49 years, and 50+ years.
  • To determine if clinical vaccine effectiveness differs for receipt of 1 vs. 2 doses of monovalent nH1N1 vaccine
  • To evaluate the potential for effect modification of vaccination by the presence of chronic medical conditions
  • To evaluate the possible effect of vaccination on reducing the length of hospitalization, given admission with RT-PCR-confirmed nH1N1 (decreased severity)
  • To evaluate cross-protection for nH1N1 and seasonal influenza vaccines


In July 2009, ACIP recommended that all persons aged 6 months to 24 years be vaccinated with nH1N1 vaccine. Persons aged 25 through 64 years who have health conditions associated with higher risk of medical complications from influenza, and all pregnant women are also recommended to receive nH1N1 vaccine. Once demand for vaccine for prioritized groups has been met at the local level, it is expected that vaccination of other persons will follow.

The methods for the nH1N1 vaccine effectiveness study are similar to those of the adult influenza vaccine effectiveness study. Laboratory-confirmed influenza reports regarding New Haven County residents are received from the Connecticut Department of Public Health. These laboratory-confirmed cases are followed up by contacting hospital infection control practitioners (ICPs) to identify cases of influenza-related hospitalizations at the ICPs' respective hospitals. For each hospitalized case enrolled, two age-matched, community-dwelling controls who were not hospitalized with an influenza infection are recruited. Cases and controls are contacted by telephone. After obtaining consent for participation, study personnel interview each enrollee regarding influenza vaccine status, chronic medical conditions, and other risk factors for influenza complications. Additionally, health care providers are contacted for both cases and controls to obtain immunization and medical history. Data collection for this case-control study is now complete.

Related Links

Thompson MG, LZ Sokolow, O Almendares, K Openo, MM Farley, J Meek, et al. 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. Clinical Infectious Diseases. 2013 Dec;57(11):1587-92.