Age Distribution of Cases of 2009 (H1N1) Pandemic Influenza in Comparison with Seasonal Influenza

Abstract

IntroductionSeveral 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.MethodsWe 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.ResultsFor 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].ConclusionsThe 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.

Pandemic H1N1 influenza surveillance in Haiti, July–December 2009

“…We found that in Haiti young adults and children were the predominant age groups affected by pandemic influenza. This finding is consistent with demographic data reported in other countries in the region, including the United States and Barbados, where most cases of pandemic (H1N1) 2009 were found in children and young adults 7 , 8 …”

Inference of seasonal and pandemic influenza transmission dynamics

“…Susceptibility at the beginning of the spring 2009 pandemic wave, although significantly higher than any of the epidemic seasons, is only 75.6% (72.7-78.8%). It is not surprising to find susceptibility lower than 100%, as the elderly are often less susceptible to a pandemic strain due to prior exposure to structurally similar strains (19,35,36). However, initial susceptibility, commonly assumed to decrease over time because a portion of the population would have been infected during the herald wave, is even higher at the beginning of the fall 2009 wave (Fig.…”

A Subregional Analysis of Epidemiologic and Genetic Characteristics of Influenza A(H1N1)pdm09 in Africa: Senegal, Cape Verde, Mauritania, and Guinea, 2009–2010

“…This finding is consistent (excepted for elders groups) with other studies, in which the greatest increase in influenza cases during 2009 (H1N1) pandemic influenza period was seen for schoolaged children, adolescents, and younger adults. 18,19 Considering the elderly, the A(H1N1)pdm09 detection rate (2/30; 6.6%; P value = 0.76) ( Table 3) is because of the very small number of elderly people in consultation for ILI in these countries during the 2009-2010 period (0.9%). In fact, previous studies have shown that the elderly have better protective immunity acquired during previous contact with the virus.…”