Abstract
Following the global spread of pandemic influenza A(H1N1)2009, several pandemic vaccines have been rapidly developed. The United Kingdom and many other countries in the northern hemisphere implemented seasonal and pandemic influenza vaccine programmes in October 2009. We present the results of a case–control study to estimate effectiveness of such vaccines in preventing confirmed pandemic influenza infection. Some 5,982 individuals with influenza-like illness seen in general practices between November 2009 and January 2010 were enrolled. Those testing positive on PCR for pandemic influenza were assigned as cases and those testing negative as controls. Vaccine effectiveness was estimated as the relative reduction in odds of confirmed infection between vaccinated and unvaccinated individuals. Fourteen or more days after immunisation with the pandemic vaccine, adjusted vaccine effectiveness (VE) was 72% (95% confidence interval (CI): 21% to 90%). If protection was assumed to start after seven or more days, the adjusted VE was 71% (95% CI: 37% to 87%). Pandemic influenza vaccine was highly effective in preventing confirmed infection with pandemic influenza A(H1N1)2009 from one week after vaccination. No evidence of effectiveness against pandemic influenza A(H1N1)2009 was found for the 2009/10 trivalent seasonal influenza vaccine (adjusted VE of -30% (95% CI: -89% to 11%)).
Highlights
Following the emergence and rapid global spread of pandemic influenza A(H1N1)2009 virus in April 2009 [1], several vaccines against this virus were quickly developed [2,3,4,5,6]
There are, potential limitations: Firstly, a convenience sample was used because random sampling of patients for a routine surveillance system based on general practitioner (GP)-provided care is not feasible
That the sampling would have caused substantial bias: it is conceivable that a GP might selectively sample patients based on their vaccination status, their case or control status would not have been known at the time of sampling
Summary
Following the emergence and rapid global spread of pandemic influenza A(H1N1)2009 virus in April 2009 [1], several vaccines against this virus were quickly developed [2,3,4,5,6]. Clinical trials, including products with a new squalene adjuvant (MF59 or AS03) demonstrated that these novel pandemic vaccines were immunogenic in various target populations [2,3,4,5,6]. The United Kingdom (UK), as many other countries in the northern hemisphere, implemented its seasonal and pandemic influenza vaccine programmes in autumn 2009. The pandemic vaccine programme was initially targeted at clinical risk groups older than six months, pregnant women and healthcare workers [12] and later extended to all healthy children six months to five years of age. Pandemrix was the main vaccine administered through the UK pandemic vaccine programme: by late February 2010, provisional uptake for the first dose of Pandemrix in England was 37.1% for clinical at-risk groups, 20.4% for healthy children six months to five years of age and 39.9% for healthcare workers [13]
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