Abstract
During the 2009/10 influenza A(H1N1)pdm09 pandemic, the five Nordic countries adopted different approaches to pandemic vaccination. We compared pandemic vaccination strategies and severe influenza outcomes, in seasons 2009/10 and 2010/11 in these countries with similar influenza surveillance systems. We calculated the cumulative pandemic vaccination coverage in 2009/10 and cumulative incidence rates of laboratory confirmed A(H1N1)pdm09 infections, intensive care unit (ICU) admissions and deaths in 2009/10 and 2010/11. We estimated incidence risk ratios (IRR) in a Poisson regression model to compare those indicators between Denmark and the other countries. The vaccination coverage was lower in Denmark (6.1%) compared with Finland (48.2%), Iceland (44.1%), Norway (41.3%) and Sweden (60.0%). In 2009/10 Denmark had a similar cumulative incidence of A(H1N1)pdm09 ICU admissions and deaths compared with the other countries. In 2010/11 Denmark had a significantly higher cumulative incidence of A(H1N1)pdm09 ICU admissions (IRR: 2.4; 95% confidence interval (CI): 1.9-3.0) and deaths (IRR: 8.3; 95% CI: 5.1-13.5). Compared with Denmark, the other countries had higher pandemic vaccination coverage and experienced less A(H1N1)pdm09-related severe outcomes in 2010/11. Pandemic vaccination may have had an impact on severe influenza outcomes in the post-pandemic season. Surveillance of severe outcomes may be used to compare the impact of influenza between seasons and support different vaccination strategies.
Highlights
In 2009, the World Health Organization recommended adjuvanted vaccines in response to the A(H1N1)pdm09 pandemic [1]
We evaluated how the pandemic and the post-pandemic influenza seasons progressed in the Nordic countries and present the results in light of the different vaccination strategies used
Our observational study allowed a comprehensive description of timing and coverage of the pandemic vaccinations and severe outcomes of influenza A(H1N1)
Summary
In 2009, the World Health Organization recommended adjuvanted vaccines in response to the A(H1N1)pdm pandemic [1]. The five Nordic countries (Denmark, Finland, Iceland, Norway and Sweden) all used the monovalent AS03-adjuvanted pandemic influenza vaccine Pandemrix [2]. Several studies have estimated the effectiveness of the pandemic vaccine in preventing A(H1N1)pdm during the pandemic [3,4,5,6,7]. It is possible that a high pandemic vaccination coverage in a population would affect the distribution of circulating influenza subtypes and disease severity for a longer period after a pandemic. We are not aware of any studies that assessed how different pandemic vaccination strategies may have affected the influenza type/subtype distribution and the epidemiology of severe influenza in the post-pandemic season
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