title: Twin Peaks: A/H1N1 Pandemic Influenza Virus Infection and Vaccination in Norway, 2009-2010.
abstract: BACKGROUND
  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.
  
  
  METHODS
  A 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.
  
  
  RESULTS
  The 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.
  
  
  CONCLUSIONS
  This 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.
authors.affiliation: Global Epidemiology, GSK Vaccines, Wavre, Belgium.
authors.forename: Thierry
authors.initials: T
authors.lastname: Van Effelterre
authors.affiliation: Business & Decision Life Sciences (on behalf of GSK Vaccines), Brussels, Belgium.
authors.forename: Gaël
authors.initials: G
authors.lastname: Dos Santos
authors.affiliation: GSK Vaccines, King of Prussia, PA, United States of America.
authors.forename: Vivek
authors.initials: V
authors.lastname: Shinde
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control* / Q000635:transmission; D008875:Middle Aged; D008954:Models, Biological; D009664:Norway / Q000453:epidemiology; D012621:Seasons; D014611:Vaccination; D055815:Young Adult