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

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