title: Burden of illness of the 2009 pandemic of influenza A (H1N1) in Denmark.
abstract: We analysed Danish surveillance data to estimate influenza-associated morbidity and mortality in 2009. To obtain population-based estimates of the clinical attack rate, we combined data from two different primary health care surveillance systems, national numbers of the proportion of positive influenza tests, and data from a web-based interview on health care seeking behaviour during the pandemic. From a national registry, we obtained data on hospital admissions (ICD-10 codes) for influenza related conditions. Admission to intensive care was monitored by a dedicated surveillance scheme. Mortality was estimated among laboratory confirmed cases but was also expressed as excess all-cause mortality attributed to influenza-like illness in a multivariable time series analysis. In total, we estimated that 274,000 individuals (5%) in Denmark experienced clinical illness. The highest attack rate was found in children 5-14 years (15%). Compared with the expected number of hospital admissions, there was an 80% increase in number of influenza related hospital admissions in this age group. The numbers of patients admitted to intensive care approached 5% of the national capacity. Estimates of the number of deaths ranged from 30 to 312 (0.5-5.7 per 100,000 population) depending on the methodology. In conclusion, the pandemic was characterised by high morbidity and unprecedented high rates of admissions to hospitals for a range of influenza-related conditions affecting mainly children. Nonetheless, the burden of illness was lower than assumed in planning scenarios, and the present pandemic compares favourable with the 20th century pandemics.
authors.affiliation: Department of Epidemiology, Statens Serum Institut, Copenhagen, Denmark.
authors.forename: Kåre
authors.initials: K
authors.lastname: Mølbak
authors.forename: Katarina
authors.initials: K
authors.lastname: Widgren
authors.forename: Klaus Skovbo
authors.initials: KS
authors.lastname: Jensen
authors.forename: Steen
authors.initials: S
authors.lastname: Ethelberg
authors.forename: Peter H
authors.initials: PH
authors.lastname: Andersen
authors.forename: Annette H
authors.initials: AH
authors.lastname: Christiansen
authors.forename: Hanne-Dorthe
authors.initials: HD
authors.lastname: Emborg
authors.forename: Sophie
authors.initials: S
authors.lastname: Gubbels
authors.forename: Katja Majlund
authors.initials: KM
authors.lastname: Harder
authors.forename: Tyra Grove
authors.initials: TG
authors.lastname: Krause
authors.forename: Anne
authors.initials: A
authors.lastname: Mazick
authors.forename: Lars Peter
authors.initials: LP
authors.lastname: Nielsen
authors.forename: Jens
authors.initials: J
authors.lastname: Nielsen
authors.forename: Palle
authors.initials: P
authors.lastname: Valentiner-Branth
authors.forename: Steffen
authors.initials: S
authors.lastname: Glismann
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D002648:Child; D002675:Child, Preschool; D017281:Cost of Illness; D003718:Denmark / Q000453:epidemiology; D005260:Female; D006760:Hospitalization; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000453:epidemiology*; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D012621:Seasons; D055815:Young Adult