Registry-based surveillance of influenza-associated hospitalisations during the 2009 influenza pandemic in Denmark: the hidden burden on the young

Abstract

Background: To follow the impact of the 2009 influenza pandemic in Denmark, influenza surveillance was extended with a system monitoring potentially influenza-associated hospitalisations.

Methodology/principal findings: National administrative data from 2004-2010 from the automatic reporting of all hospital visits and admissions in Denmark (population 5.5 million) were used. In-patient hospitalisations linked to ICD-10 codes for potentially influenza-associated conditions (influenza, viral and bacterial pneumonia, respiratory distress, and febrile convulsion) were aggregated by week and age groups; <5 years, 5-24 years, 25-64 years and ≥65 years. Weekly numbers of influenza-associated hospitalisations were plotted to follow the course of the pandemic. We calculated the total numbers of influenza-associated hospitalisations in each influenza season (week 30 to week 15, the following year). Risk ratios of being admitted with an influenza-associated condition in this season (2009/2010) compared to the previous five seasons (2004/2005-2008/2009) were calculated using binary regression. During the pandemic season, influenza-associated hospitalisations peaked in week 47, 2009. The total number of influenza-associated hospitalisations was 38,273 compared to the median of previous seasons of 35,662 (p = 0.28). The risk ratio of influenza-associated hospitalisations during the pandemic season compared to previous seasons was 1.63 (95%CI 1.49-1.78) for 5-24 year-olds and ranged between 0.98 and 1.08 for the other three age groups.

Conclusions: The 2009 pandemic influenza did not lead to an overall increase in the number of influenza-associated hospitalisations in Denmark in the 2009/2010 season and could be managed within existing hospital capacity. However, there was a disproportionally large impact on the age group 5-24 years. The influenza-associated hospitalisations during the 2009/2010 pandemic influenza season bore the signature features of historical pandemics: A skewed age-pattern and early out of season transmission.

Figure 1. Time-series of weekly numbers of influenza-associated hospitalisations in 2004–2010, by age group, in Denmark.

The baseline with 95% upper confidence interval is fitted to the pre-pandemic data; week 1, 2004 until week 17, 2009. The peak week of the 2009 influenza pandemic, week 47 of 2009, is indicated with vertical lines.

Figure 2. Cumulative incidence and risk ratios of influenza-associated hospitalisations during the 2009 influenza pandemic in Denmark.

The cumulative incidence (bars) of influenza-associated hospitalisations for week 30 of 2009 to week 15 of 2010, by 5-year age groups, and the risk ratios (line) for influenza-associated hospitalisations in the pandemic season compared to the five previous seasons adjusted for an optional underlying trend in hospitalisations. In the age range 5 to 49 years, the risk ratios of hospitalisations were significantly higher than in the previous five seasons (estimates marked with diamonds).