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Comparative Study
. 2015 Aug 15;61(4):563-71.
doi: 10.1093/cid/civ345. Epub 2015 May 4.

Differences in the Epidemiology of Human Cases of Avian Influenza A(H7N9) and A(H5N1) Viruses Infection

Ying Qin  1 , Peter W Horby  2 , Tim K Tsang  3 , Enfu Chen  4 , Lidong Gao  5 , Jianming Ou  6 , Tran Hien Nguyen  7 , Tran Nhu Duong  7 , Viktor Gasimov  8 , Luzhao Feng  1 , Peng Wu  3 , Hui Jiang  1 , Xiang Ren  1 , Zhibin Peng  1 , Sa Li  1 , Ming Li  1 , Jiandong Zheng  1 , Shelan Liu  4 , Shixiong Hu  5 , Rongtao Hong  6 , Jeremy J Farrar  9 , Gabriel M Leung  3 , George F Gao  10 , Benjamin J Cowling  3 , Hongjie Yu  1
Affiliations
Comparative Study

Differences in the Epidemiology of Human Cases of Avian Influenza A(H7N9) and A(H5N1) Viruses Infection

Ying Qin et al. Clin Infect Dis. .

Abstract

Background: The pandemic potential of avian influenza viruses A(H5N1) and A(H7N9) remains an unresolved but critically important question.

Methods: We compared the characteristics of sporadic and clustered cases of human H5N1 and H7N9 infection, estimated the relative risk of infection in blood-related contacts, and the reproduction number (R).

Results: We assembled and analyzed data on 720 H5N1 cases and 460 H7N9 cases up to 2 November 2014. The severity and average age of sporadic/index cases of H7N9 was greater than secondary cases (71% requiring intensive care unit admission vs 33%, P = .007; median age 59 years vs 31, P < .001). We observed no significant differences in the age and severity between sporadic/index and secondary H5N1 cases. The upper limit of the 95% confidence interval (CI) for R was 0.12 for H5N1 and 0.27 for H7N9. A higher proportion of H5N1 infections occurred in clusters (20%) compared to H7N9 (8%). The relative risk of infection in blood-related contacts of cases compared to unrelated contacts was 8.96 for H5N1 (95% CI, 1.30, 61.86) and 0.80 for H7N9 (95% CI, .32, 1.97).

Conclusions: The results are consistent with an ascertainment bias towards severe and older cases for sporadic H7N9 but not for H5N1. The lack of evidence for ascertainment bias in sporadic H5N1 cases, the more pronounced clustering of cases, and the higher risk of infection in blood-related contacts, support the hypothesis that susceptibility to H5N1 may be limited and familial. This analysis suggests the potential pandemic risk may be greater for H7N9 than H5N1.

Keywords: clinical epidemiology; cluster; influenza A(H5N1); influenza A(H7N9).

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Figures

Figure 1.
Figure 1.
Epidemic curve of sporadic and clustered human cases with H5N1 and H7N9 virus infection (as of 2 November 2014). A, Number of sporadic and clustered human cases with H5N1virus infection by month of illness onset. B, Number of sporadic and clustered human cases with H7N9 virus infection by week of illness onset. Note for A, When the date of illness onset is missing, the earliest date among the date of hospitalization, date of outcome, and date of World Health Organization (WHO) report is used. The month of illness onset for 23 cases in total 720 cases are missing and excluded from this epidemic curve: 21 cases of Indonesia in 2009 and 2 cases of Turkey in 2006.
Figure 2.
Figure 2.
Proportion of cases occurring in household clusters by the probability of infection given exposure.
See this image and copyright information in PMC

Comment in

References

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