Abstract
BackgroundIn this study, we assess how effective pandemic and trivalent 2009-2010 seasonal vaccines were in preventing influenza-like illness (ILI) during the 2009 A(H1N1) pandemic in France. We also compare vaccine effectiveness against ILI versus laboratory-confirmed pandemic A(H1N1) influenza, and assess the possible bias caused by using non-specific endpoints and observational data.Methodology and Principal FindingsWe estimated vaccine effectiveness by using the following formula: VE = (PPV-PCV)/(PPV(1-PCV)) × 100%, where PPV is the proportion vaccinated in the population and PCV the proportion of vaccinated influenza cases. People were considered vaccinated three weeks after receiving a dose of vaccine. ILI and pandemic A(H1N1) laboratory-confirmed cases were obtained from two surveillance networks of general practitioners. During the epidemic, 99.7% of influenza isolates were pandemic A(H1N1). Pandemic and seasonal vaccine uptakes in the population were obtained from the National Health Insurance database and by telephonic surveys, respectively. Effectiveness estimates were adjusted by age and week. The presence of residual biases was explored by calculating vaccine effectiveness after the influenza period. The effectiveness of pandemic vaccines in preventing ILI was 52% (95% confidence interval: 30–69) during the pandemic and 33% (4–55) after. It was 86% (56–98) against confirmed influenza. The effectiveness of seasonal vaccines against ILI was 61% (56–66) during the pandemic and 19% (−10–41) after. It was 60% (41–74) against confirmed influenza.ConclusionsThe effectiveness of pandemic vaccines in preventing confirmed pandemic A(H1N1) influenza on the field was high, consistently with published findings. It was significantly lower against ILI. This is unsurprising since not all ILI cases are caused by influenza. Trivalent 2009-2010 seasonal vaccines had a statistically significant effectiveness in preventing ILI and confirmed pandemic influenza, but were not better in preventing confirmed pandemic influenza than in preventing ILI. This lack of difference might be indicative of selection bias.
Highlights
Estimating the field effectiveness of influenza vaccines (VE) poses specific challenges for the 2009 A(H1N1) pandemic
The effectiveness of pandemic vaccines in preventing confirmed pandemic A(H1N1) influenza on the field was high, consistently with published findings. It was significantly lower against influenza-like illness (ILI). This is unsurprising since not all ILI cases are caused by influenza
Sentinelles general practitioners (GPs) reported 7586 ILI cases above 6 month-old, with information on age and pandemic vaccine status, which were included in the pandemic vaccines analysis
Summary
Estimating the field effectiveness of influenza vaccines (VE) poses specific challenges for the 2009 A(H1N1) pandemic. On November 12th (week 46) all other health care professional were called (880,000), as well as all persons in contact with infants younger than six month-old (1,200,000), childminders working with children under three year-old (500,000), and every person between six months and 64 years of age with a risk factor (2,815,000). Pregnant women from their second trimester and 6- to 23 month-old children without risk factor were called on November 20th (week 47). We compare vaccine effectiveness against ILI versus laboratory-confirmed pandemic A(H1N1) influenza, and assess the possible bias caused by using non-specific endpoints and observational data
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
