Age distribution of cases of 2009 (H1N1) pandemic influenza in comparison with seasonal influenza

Abstract

Introduction: Several aspects of the epidemiology of 2009 (H1N1) pandemic influenza have not been accurately determined. We sought to study whether the age distribution of cases differs in comparison with seasonal influenza.

Methods: We searched for official, publicly available data through the internet from different countries worldwide on the age distribution of cases of influenza during the 2009 (H1N1) pandemic influenza period and most recent seasonal influenza periods. Data had to be recorded through the same surveillance system for both compared periods.

Results: For 2009 pandemic influenza versus recent influenza seasons, in USA, visits for influenza-like illness to sentinel providers were more likely to involve the age groups of 5-24, 25-64 and 0-4 years compared with the reference group of >64 years [odds ratio (OR) (95% confidence interval (CI)): 2.43 (2.39-2.47), 1.66 (1.64-1.69), and 1.51 (1.48-1.54), respectively]. Pediatric deaths were less likely in the age groups of 2-4 and <2 years than the reference group of 5-17 years [OR (95% CI): 0.46 (0.25-0.85) and 0.49 (0.30-0.81), respectively]. In Australia, notifications for laboratory-confirmed influenza were more likely in the age groups of 10-19, 5-9, 20-44, 45-64 and 0-4 years than the reference group of >65 years [OR (95% CI): 7.19 (6.67-7.75), 5.33 (4.90-5.79), 5.04 (4.70-5.41), 3.12 (2.89-3.36) and 1.89 (1.75-2.05), respectively]. In New Zealand, consultations for influenza-like illness by sentinel providers were more likely in the age groups of <1, 1-4, 35-49, 5-19, 20-34 and 50-64 years than the reference group of >65 years [OR (95% CI): 2.38 (1.74-3.26), 1.99 (1.62-2.45), 1.57 (1.30-1.89), 1.57 (1.30-1.88), 1.40 (1.17-1.69) and 1.39 (1.14-1.70), respectively].

Conclusions: The greatest increase in influenza cases during 2009 (H1N1) pandemic influenza period, in comparison with most recent seasonal influenza periods, was seen for school-aged children, adolescents, and younger adults.

Figure 1. Percentile distribution by age group of notifications for laboratory-confirmed influenza in Australia, during 2009 (H1N1) pandemic influenza and the 3 previous influenza seasons combined.

Figure 2. Adjusted sentinel average weekly consultation rate for influenza-like illness by age group in New Zealand, during 2009 (H1N1) pandemic influenza and the previous influenza season.

Consultation rate by age group is shown as times X consultation rate for total population.

Figure 3. Percentile distribution by age group of visits for influenza-like-illness to sentinel providers, in the United States of America, during 2009 (H1N1) pandemic influenza and the 2 previous influenza seasons combined.

Figure 4. Percentile distribution by age group of pediatric deaths associated with laboratory-confirmed influenza, in the United States of America, during 2009 (H1N1) pandemic influenza and 2 previous influenza seasons combined.