The first, second and third wave of pandemic influenza A (H1N1)pdm09 in North Denmark Region 2009–2011: a population-based study of hospitalizations

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Background and objectives

Denmark experienced three waves of the new pandemic influenza A (H1N1)pdm09 from July 2009 to February 2011. The aim of the study was to describe the epidemiology and clinical characteristics of hospitalized patients in a defined population of North Denmark Region with a mixed urban and rural community of 579 000 inhabitants.


Review of medical records of all hospitalized patients with confirmed influenza A from July 2009 to February 2011.


Two hundred and seventy-three patients were admitted to hospital. The age-related population incidences of hospitalization were as follows: 0–14 years: 111/100 000, 15–64 years: 39/100 000, and ≥65 years: 17/100 000. During the first wave (July 2009–August 2009), three patients were admitted – none received treatment in intensive care units (ICUs), during the second wave (October 2009–January 2010), 158 patients were admitted – nine received treatment in ICUs, and during the third wave (December 2010–February 2011), 112 patients were admitted – 25 received treatment in ICUs. Fourteen patients (5%) died within 30 days of diagnosis (median 55 years; range 14–76 years) and additional seven patients (2·6%) died within 365 days (median 25 years; range 1–86 years).


Patients hospitalized with pandemic influenza A (H1N1)pdm09 were predominantly children and younger adults, and only a few patients were >65 years. The third wave was the most severe taking the number and percentage of patients admitted to ICUs and 30-day mortality into consideration. We observed that the incidence of hospitalizations as well as clinical severity among younger adults did not decline from the second to the third wave.

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