Effectiveness of H1N1 vaccine for the prevention of pandemic influenza in Scotland, UK: a retrospective observational cohort study

Background A targeted vaccination programme for pandemic H1N1 2009 influenza was introduced in Scotland, UK, in October, 2009. We sought to assess the effectiveness of this vaccine in a sample of the Scottish population during the 2009-10 pandemic.

Methods We assessed the effectiveness of the Scottish pandemic H1N1 2009 influenza vaccination with a retrospective cohort design. We linked data of patient-level primary care, hospital records, death certification, and virological swabs to construct our cohort. We estimated vaccine effectiveness in a nationally representative sample of the Scottish population by establishing the risk of hospital admission and death (adjusted for potential confounders) resulting from influenza-related morbidity in vaccinated and unvaccinated patients and laboratory-confirmed cases of influenza H1N1 2009 in a subset of patients.

Findings Pandemic H1N1 2009 influenza vaccination began in week 43 of 2009 (Oct 21, 2009) and was given to 38 296 (15.5%, 95% CI 15.4-15. 6) of 247 178 people by the end of the study period (Jan 31, 2010). 208 882 (85%) people were unvaccinated. There were 5207 emergency hospital admissions and 579 deaths in the unvaccinated population and 924 hospital admissions and 71 deaths in the vaccinated population during 23 893 359 person-days of observation. The effectiveness of H1N1 vaccination for prevention of emergency hospital admissions from influenza-related disorders was 19.5% (95% CI 08-34.7). The vaccine's effectiveness in preventing laboratory-confirmed influenza was 77.0% (95% CI 2.0-95.0).

Interpretation Pandemic H1N1 2009 influenza vaccination was associated with protection against pandemic influenza and a reduction in hospital admissions from influenza-related disorders in Scotland during the 2009-10 pandemic.