Differences in the Epidemiology of Human Cases of Avian Influenza A(H7N9) and A(H5N1) Viruses Infection.
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.
Division of Infectious Disease, Key Laboratory of Surveillance and Early-warning on Infectious Disease, Chinese Center for Disease Control and Prevention, Beijing, China.
Ying
Y
Qin
Oxford University Clinical Research Unit, Wellcome Trust Major Overseas Programme, Ho Chi Minh City, Vietnam Centre for Tropical Medicine, Nuffield Department of Clinical Medicine, Oxford University, United Kingdom Singapore Infectious Disease Initiative.
Peter W
PW
Horby
WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region.
Tim K
TK
Tsang
Zhejiang Provincial Center for Disease Control and Prevention, Hangzhou.
Enfu
E
Chen
Hunan Provincial Center for Disease Control and Prevention, Changsha.
Lidong
L
Gao
Fujian Provincial Center for Disease Control and Prevention, Fuzhou, China.
Jianming
J
Ou
National Institute of Hygiene and Epidemiology, Hanoi, Vietnam.
Tran Hien
TH
Nguyen
National Institute of Hygiene and Epidemiology, Hanoi, Vietnam.
Tran Nhu
TN
Duong
Ministry of Health of Azerbaijan, Baku.
Viktor
V
Gasimov
Division of Infectious Disease, Key Laboratory of Surveillance and Early-warning on Infectious Disease, Chinese Center for Disease Control and Prevention, Beijing, China.
Luzhao
L
Feng
WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region.
Peng
P
Wu
Division of Infectious Disease, Key Laboratory of Surveillance and Early-warning on Infectious Disease, Chinese Center for Disease Control and Prevention, Beijing, China.
Hui
H
Jiang
Division of Infectious Disease, Key Laboratory of Surveillance and Early-warning on Infectious Disease, Chinese Center for Disease Control and Prevention, Beijing, China.
Xiang
X
Ren
Division of Infectious Disease, Key Laboratory of Surveillance and Early-warning on Infectious Disease, Chinese Center for Disease Control and Prevention, Beijing, China.
Zhibin
Z
Peng
Division of Infectious Disease, Key Laboratory of Surveillance and Early-warning on Infectious Disease, Chinese Center for Disease Control and Prevention, Beijing, China.
Sa
S
Li
Division of Infectious Disease, Key Laboratory of Surveillance and Early-warning on Infectious Disease, Chinese Center for Disease Control and Prevention, Beijing, China.
Ming
M
Li
Division of Infectious Disease, Key Laboratory of Surveillance and Early-warning on Infectious Disease, Chinese Center for Disease Control and Prevention, Beijing, China.
Jiandong
J
Zheng
Zhejiang Provincial Center for Disease Control and Prevention, Hangzhou.
Shelan
S
Liu
Hunan Provincial Center for Disease Control and Prevention, Changsha.
Shixiong
S
Hu
Fujian Provincial Center for Disease Control and Prevention, Fuzhou, China.
Rongtao
R
Hong
Oxford University Clinical Research Unit, Wellcome Trust Major Overseas Programme, Ho Chi Minh City, Vietnam Centre for Tropical Medicine, Nuffield Department of Clinical Medicine, Oxford University, United Kingdom Singapore Infectious Disease Initiative ISARIC, Centre for Tropical Medicine, University of Oxford, Churchill Hospital, United Kingdom.
Jeremy J
JJ
Farrar
WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region.
Gabriel M
GM
Leung
CAS Key Laboratory of Pathogenic Microbiology and Immunology, Institute of Microbiology, Chinese Academy of Sciences Office of Director-General, Chinese Center for Disease Control and Prevention, Beijing, China.
George F
GF
Gao
WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region.
Benjamin J
BJ
Cowling
Division of Infectious Disease, Key Laboratory of Surveillance and Early-warning on Infectious Disease, Chinese Center for Disease Control and Prevention, Beijing, China.
Hongjie
H
Yu
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D050936:Basic Reproduction Number; D002648:Child; D002675:Child, Preschool; D005192:Family Health; D005260:Female; D006801:Humans; D007223:Infant; D053124:Influenza A Virus, H5N1 Subtype / Q000302:isolation & purification*; D064766:Influenza A Virus, H7N9 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000473:pathology / Q000821:virology*; D008297:Male; D008875:Middle Aged; D018570:Risk Assessment; D012720:Severity of Illness Index; D055815:Young Adult
clinical epidemiology; cluster; influenza A(H5N1); influenza A(H7N9)