Vaccine effectiveness in pandemic influenza - primary care reporting (VIPER): an observational study to assess the effectiveness of the pandemic influenza A (H1N1)v vaccine.

OBJECTIVES

To determine influenza A (H1N1)v vaccine effectiveness (VE) in the Scottish population at an early stage of the 2009-10 H1N1v vaccination programme, using a sentinel surveillance network of 41 general practices contributing to the Practice Team Information (PTI) network.

METHODS

Retrospective cohort study using record linkage. Using the Community Health Index (CHI) number, general practice patient-level data were extracted and linked to the Scottish Morbidity Record (SMR) catalogue, containing information on all inpatient hospitalisations in Scotland. The Health Protection Scotland (HPS) data set was also used, consisting of laboratory-confirmed cases of influenza A (H1N1)v from the practices. The study involved a longitudinal evaluation of the aspect of the influenza A (H1N1)v vaccination programme implemented through general practice in autumn/winter 2009.

RESULTS

At 25 December 2009, vaccine uptake estimates for the study population were 12.0% (95% CI 11.9 to 12.1). For those patients in an at-risk group (n = 59,721), the uptake rate was 37.5% (95% CI 37.1 to 37.9). Among the 1492 patients swabbed, 467 were positive for H1N1, giving a positivity rate of 31.3% [95% confidence interval (CI) 29.0 to 33.7]. Among those in a clinical risk group who were not vaccinated, 41.3% (95% CI 35.6 to 46.9) tested positive for influenza A (H1N1)v, a significant difference from the H1N1 positivity percentage among patients with no clinical risk (p < 0.01). Among those vaccinated and in a clinical risk group, only one patient (5%, 95% CI 0.3 to 23.6) tested after vaccination was positive for influenza A (H1N1)v. By comparing postvaccination swabs in those who were vaccinated with swabs taken in those who remained unvaccinated, the VE was found to be 95.0% (95% CI 76.0 to 100.0). In the study population there were 2739 admissions to hospital, of which 1241 were emergency admissions; all 48 emergency hospitalisations for influenza and pneumonia occurred in patients who did not receive the vaccine. VE for single or combined end points of influenza and pneumonia hospitalisation for all patients was estimated at 100.0% (95% CI infinity to 100.0). There were 132 hospitalisations in the unvaccinated group versus five in the vaccinated group for cardiovascular-related conditions. There were 193 hospitalisations in the unvaccinated group versus nine in those vaccinated in the group of patients admitted for influenza, pneumonia, chronic obstructive pulmonary disease (COPD) and cardiovascular-related conditions. VE for cardiovascular-related conditions alone, or in individuals with influenza, pneumonia COPD and cardiovascular-related conditions, was 71.1% (95% CI 11.3 to 90.6) and 64.7% (95% CI 12.0 to 85.8) respectively.

CONCLUSIONS

Evidence from swabs submitted from patients in the cohort who presented in general practice with influenza-like illness suggests that the introduction of influenza A (H1N1)v vaccine in Scotland during 2009 was associated with a high degree of protection. Influenza A (H1N1)v immunisation in primary health-care settings appears to be both effective and widely acceptable, and should continue to be the mainstay of disease prevention for at-risk patients. A further analysis encompassing the whole influenza season is required to cover more days of vaccination exposure and increase precision.