Effect of vaccines and antivirals during the major 2009 A(H1N1) pandemic wave in Norway--and the influence of vaccination timing

• Published in: PloS one Vol. 7; no. 1; p. e30018
• Main Authors: Blasio, Birgitte Freiesleben de, Iversen, Bjørn G, Tomba, Gianpaolo Scalia
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 10.01.2012; Public Library of Science (PLoS)
• Subjects: Adjuvants, Immunologic - administration & dosage; Adolescent; Adult; Age; Age Distribution; Age groups; Aged; Analysis; Antiviral agents; Antiviral Agents - therapeutic use; Basic Reproduction Number; Biocides; Biology; Child; Child, Preschool; Children; Disease control; Estimates; Health aspects; Health surveillance; Humans; Illnesses; Infant; Infant, Newborn; Infections; Infectious diseases; Influenza; Influenza A Virus, H1N1 Subtype - immunology; Influenza vaccines; Influenza Vaccines - immunology; Influenza, Human - epidemiology; Influenza, Human - immunology; Influenza, Human - prevention & control; Influenza, Human - transmission; Mathematics; Medicine; Middle Aged; Models, Biological; Norway - epidemiology; Pandemics; Pandemics - prevention & control; Pediatric diseases; Population; Prevalence; Public health; Respiratory diseases; Sensitivity and Specificity; Social and Behavioral Sciences; Swine flu; Time Factors; Vaccination; Vaccines; Viruses; Young Adult; Norway; United Kingdom > UK; United States > US; Europe; England; France; Germany
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0030018

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.