Burden of illness of the 2009 pandemic of influenza A (H1N1) in Denmark

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 influe...

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Bibliographic Details
Published in:Vaccine 2011-07, Vol.29 (S2), p.B63-B69
Main Authors: Mølbak, Kåre, Widgren, Katarina, Jensen, Klaus Skovbo, Ethelberg, Steen, Andersen, Peter H, Christiansen, Annette H, Emborg, Hanne-Dorthe, Gubbels, Sophie, Harder, Katja Majlund, Krause, Tyra Grove, Mazick, Anne, Nielsen, Lars Peter, Nielsen, Jens, Valentiner-Branth, Palle, Glismann, Steffen
Format: Article
Language:English
Subjects:
Adolescent
Adult
Age
Age Distribution
Aged
Allergy and Immunology
burden of disease
Burden of illness
Child
Child, Preschool
Children
Cost of Illness
Data processing
Denmark - epidemiology
Female
Health care
health services
Hospitalization
Hospitals
Humans
Illnesses
Infant
Influenza
Influenza A
Influenza A Virus, H1N1 Subtype
Influenza, Human - epidemiology
Intensive care
Internet
Male
Middle Aged
monitoring
Morbidity
Mortality
pandemic
Pandemic influenza A (H1N1)
Pandemics
patients
planning
Seasons
time series analysis
vaccines
Young Adult
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Summary: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.
ISSN:0264-410X
1873-2518
DOI:10.1016/j.vaccine.2011.03.064