Abstract
BackgroundVaccination campaigns against A/H1N1 2009 pandemic influenza virus (A/H1N1p) began in autumn 2009 in Europe, after the declaration of the pandemic at a global level. This study aimed to estimate the proportion of individuals vaccinated against A/H1N1p in Norway who were already infected (asymptomatically or symptomatically) by A/H1N1p before vaccination, using a mathematical model.MethodsA dynamic, mechanistic, mathematical model of A/H1N1p transmission was developed for the Norwegian population. The model parameters were estimated by calibrating the model-projected number of symptomatic A/H1N1p cases to the number of laboratory-confirmed A/H1N1p cases reported to the surveillance system, accounting for potential under-reporting. It was assumed in the base case that the likelihood of vaccination was independent of infection/disease state. A sensitivity analysis explored the effects of four scenarios in which current or previous symptomatic A/H1N1p infection would influence the likelihood of being vaccinated.ResultsThe number of model-projected symptomatic A/H1N1p cases by week during the epidemic, accounting for under-reporting and timing, closely matched that of the laboratory-confirmed A/H1N1p cases reported to the surveillance system. The model-projected incidence of symptomatic A/H1N1p infection was 27% overall, 55% in people <10 years old and 41% in people 10–20 years old. The model-projected percentage of individuals vaccinated against A/H1N1p who were already infected with A/H1N1p before being vaccinated was 56% overall, 62% in people <10 years old and 66% in people 10–20 years old. The results were sensitive to assumptions about the independence of vaccination and infection; however, even when current or previous symptomatic A/H1N1p infection was assumed to reduce the likelihood of vaccination, the estimated percentage of individuals who were infected before vaccination remained at least 32% in all age groups.ConclusionThis analysis suggests that over half the people vaccinated against A/H1N1p in Norway during the 2009 pandemic may already have been infected by A/H1N1p before being vaccinated.
Highlights
The 2009 pandemic was caused by the emergence of influenza A(H1N1)pdm09 virus, a novel strain of influenza virus A(H1N1) with a unique combination of influenza viruses genes never previously detected in animals or humans
The model parameters were estimated by calibrating the modelprojected number of symptomatic A/H1N1 pandemic influenza virus (A/H1N1p) cases to the number of laboratory-confirmed A/ H1N1p cases reported to the surveillance system, accounting for potential under-reporting
The number of symptomatic A/H1N1 2009 pandemic influenza cases by week projected by the model matched well with the number of laboratory-confirmed A/H1N1 2009 pandemic influenza cases reported through the surveillance system in all age groups combined (Fig 3A) and in individual age groups (Fig 3B–3F)
Summary
The 2009 pandemic was caused by the emergence of influenza A(H1N1)pdm virus, a novel strain of influenza virus A(H1N1) with a unique combination of influenza viruses genes never previously detected in animals or humans. The first cases were detected in Mexico and California in spring 2009, and the World Health Organization declared a pandemic (phase 6) in June 2009 [1]. The first cases of A/H1N1 2009 pandemic influenza virus in Europe were reported in late April 2009 in travellers returning from Mexico, followed by an initial wave of local transmission in spring and summer 2009, outside the normal European influenza season, and a much larger wave of transmission in autumn and winter 2009 [2], reaching a peak at around week 48 (early December) 2009 [2]. Vaccines against the A/H1N1 2009 pandemic influenza virus were rapidly developed after declaration of Phase 6 of the pandemic. Vaccination campaigns against A/H1N1 2009 pandemic influenza virus (A/H1N1p) began in autumn 2009 in Europe, after the declaration of the pandemic at a global level. This study aimed to estimate the proportion of individuals vaccinated against A/H1N1p in Norway who were already infected (asymptomatically or symptomatically) by A/H1N1p before vaccination, using a mathematical model
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