Virus-host interactions and the unusual age and sex distribution of human cases of influenza A(H7N9) in China, April 2013.

Letters Virus-host interactions and the unusual age and sex distribution of human cases of influenza A(H7N9) in China, April 2013 Over the past two weeks, Eurosurveillance has published several timely papers related to the emergence of a new influenza A(H7N9) virus affecting humans in China [1-3]. Genetic studies by Kageyama et al. [1] and Jongens et al. [2] assessed evidence in the genome for virus origin, adaptation and virulence, and a paper by Corman et al. [3] described real-time reverse-transcription PCR assays for specific virus diagnosis. While these are important aspects of novel virus characteri-sation and detection, the accrual of over 100 human cases now also affords opportunity to consider evolving epidemiologic patterns as part of population risk assessment. Perhaps the most intriguing impression to date from available surveillance findings has been the unexpected age/sex distribution of reported influenza A(H7N9) cases. The age range spans from 2 to 91 years but two thirds of influenza A(H7N9) cases have been 50 years of age or older and two thirds have been male (Table) [4,5]. Illness severity, with a substantial case fatality of 20%, shows a similar age/sex profile (Table) [4,5]. Unlike the pattern observed for influenza A(H5N1), children, both boys and girls and notably the school-aged, are under-represented among influenza A(H7N9) detections. Among the first 100 adult influenza A(H7N9) cases, men and women were equally represented in the youngest age category 20–34 years, but men were 2–3-fold more frequent than women in older age groups (Table). Furthermore, compared with women 20–34 years of age, women 50–64 and 65–79 years were each twice as frequent among influenza A(H7N9) detections. Conversely, men 50–64 and 65–79 years are each 4–5-fold more frequent among influenza A(H7N9) detections than men 20–34 years of age. While being careful not to over-interpret early surveillance data, what hypotheses might be invoked to explain that pattern? Disease occurrence is the result of the classic interaction triad of agent–host–environment. Environmental Table Human cases of influenza A(H7N9) and deaths by age group and sex, China, as of 23 April 2013 (n=109) a Age (years) <2 2–4 5–9 10–14 15–19 20–34 35–49 50–64 65–79 ≥80 Unknown Total cases 0 3 1 0 0 9 16 30 36 12 2 Female 0 0 1 0 0 4 5 9 9 4 0 Male 0 2 0 0 0 5 11 21 27 8 0 Unknown 0 1 0 0 0 0 0 0 0 …