title: Effect of vaccines and antivirals during the major 2009 A(H1N1) pandemic wave in Norway--and the influence of vaccination timing.
abstract: To evaluate the impact of mass vaccination with adjuvanted vaccines (eventually 40% population coverage) and antivirals during the 2009 influenza pandemic in Norway, we fitted an age-structured SEIR model using data on vaccinations and sales of antivirals in 2009/10 in Norway to Norwegian ILI surveillance data from 5 October 2009 to 4 January 2010. We estimate a clinical attack rate of approximately 30% (28.7-29.8%), with highest disease rates among children 0-14 years (43-44%). Vaccination started in week 43 and came too late to have a strong influence on the pandemic in Norway. Our results indicate that the countermeasures prevented approximately 11-12% of potential cases relative to an unmitigated pandemic. Vaccination was found responsible for roughly 3 in 4 of the avoided infections. An estimated 50% reduction in the clinical attack rate would have resulted from vaccination alone, had the campaign started 6 weeks earlier. Had vaccination been prioritized for children first, the intervention should have commenced approximately 5 weeks earlier in order to achieve the same 50% reduction. In comparison, we estimate that a non-adjuvanted vaccination program should have started 8 weeks earlier to lower the clinical attack rate by 50%. In conclusion, vaccination timing was a critical factor in relation to the spread of the 2009 A(H1N1) influenza. Our results also corroborate the central role of children for the transmission of A(H1N1) pandemic influenza.
authors.affiliation: Department of Biostatistics, Institute of Basic Medical Science, University of Oslo, Oslo, Norway. birgitte.deblasio@medisin.uio.no
authors.forename: Birgitte Freiesleben de
authors.initials: BF
authors.lastname: Blasio
authors.forename: Bjørn G
authors.initials: BG
authors.lastname: Iversen
authors.forename: Gianpaolo Scalia
authors.initials: GS
authors.lastname: Tomba
mesh_terms: D000276:Adjuvants, Immunologic / Q000008:administration & dosage; D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D000998:Antiviral Agents / Q000627:therapeutic use*; D050936:Basic Reproduction Number; D002648:Child; D002675:Child, Preschool; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007252:Influenza Vaccines / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology / Q000276:immunology / Q000517:prevention & control / Q000635:transmission; D008875:Middle Aged; D008954:Models, Biological; D009664:Norway / Q000453:epidemiology; D058873:Pandemics / Q000517:prevention & control*; D015995:Prevalence; D012680:Sensitivity and Specificity; D013997:Time Factors; D014611:Vaccination; D055815:Young Adult
chemical_list: D000276:Adjuvants, Immunologic; D000998:Antiviral Agents; D007252:Influenza Vaccines