title: Epidemiological and virological differences in human clustered and sporadic infections with avian influenza A H7N9.
abstract: BACKGROUND
  Previous research has suggested that avian influenza A H7N9 has a greater potential pandemic risk than influenza A H5N1. This research investigated the difference in human clustered and sporadic cases of H7N9 virus and estimated the relative risk of clustered infections.
  
  
  METHODS
  Comparative epidemiology and virology studies were performed among 72 sporadic confirmed cases, 17 family clusters (FCs) caused by human-to-human transmission, and eight live bird market clusters (LCs) caused by co-exposure to the poultry environment.
  
  
  RESULTS
  The case fatality of FCs, LCs and sporadic cases (36%, 26%, and 29%, respectively) did not differ among the three groups (p>0.05). The average age (36 years, 60 years, and 58 years), co-morbidities (31%, 60%, and 54%), exposure to birds (72%, 100%, and 83%), and H7N9-positive rate (20%, 64%, and 35%) in FCs, LCs, and sporadic cases, respectively, differed significantly (p<0.05). These higher risks were associated with increased mortality. There was no difference between primary and secondary cases in LCs (p>0.05). However, exposure to a person with confirmed avian influenza A H7N9 (primary 12% vs. secondary 95%), history of visiting a live bird market (100% vs. 59%), multiple exposures (live bird exposure and human-to-human transmission history) (12% vs. 55%), and median days from onset to antiviral treatment (6 days vs. 3 days) differed significantly between primary and secondary cases in FCs (p<0.05). Mild cases were found in 6% of primary cases vs. 32% of secondary cases in FCs (p<0.05). Twenty-five isolates from the three groups showed 99.1-99.9% homology and increased human adaptation.
  
  
  CONCLUSIONS
  There was no statistical difference in the case fatality rate and limited transmission between FCs and LCs. However, the severity of the primary cases in FCs was much higher than that of the secondary cases due to the older age and greater underlying disease of the latter patients.
authors.affiliation: Department of Respiratory Medicine, the Second Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, China.
authors.forename: Zuqun
authors.initials: Z
authors.lastname: Wu
authors.affiliation: Department of Endocrinology, The 421 Hospital of the Chinese People's Liberation Army, Guangzhou, China.
authors.forename: Jianping
authors.initials: J
authors.lastname: Sha
authors.affiliation: Department of Infectious Diseases and Key Lab of Vaccine against Hemorrhagic Fever with Renal Syndrome, Zhejiang Provincial Centre for Disease Control and Prevention, Hangzhou, Zhejiang Province, China.
authors.forename: Zhao
authors.initials: Z
authors.lastname: Yu
authors.affiliation: National Research Centre for Wildlife-Borne Diseases, Key Laboratory of Animal Ecology and Conservation Biology, Institute of Zoology, Chinese Academy of Sciences, Beijing, China.
authors.forename: Na
authors.initials: N
authors.lastname: Zhao
authors.affiliation: Department of Infectious Diseases and Key Lab of Vaccine against Hemorrhagic Fever with Renal Syndrome, Zhejiang Provincial Centre for Disease Control and Prevention, Hangzhou, Zhejiang Province, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Cheng
authors.affiliation: Research Centre for Humanities and Social Sciences, Academia Sinica, Taipei, Taiwan.
authors.forename: Ta-Chien
authors.initials: TC
authors.lastname: Chan
authors.affiliation: Department of Zoology, Faculty of Science, Kafr El Sheikh University, Kafr El Sheikh, Egypt.
authors.forename: Said
authors.initials: S
authors.lastname: Amer
authors.affiliation: School of Public Health, Shanghai Jiaotong University School of Medicine, 227 Chongqing South Road, Shanghai 200025, China. Electronic address: zhangzhiruo@sjtu.edu.cn.
authors.forename: Zhiruo
authors.initials: Z
authors.lastname: Zhang
authors.affiliation: Department of Infectious Diseases and Key Lab of Vaccine against Hemorrhagic Fever with Renal Syndrome, Zhejiang Provincial Centre for Disease Control and Prevention, Hangzhou, Zhejiang Province, China. Electronic address: liushelan@163.com.
authors.forename: Shelan
authors.initials: S
authors.lastname: Liu
mesh_terms: D000328:Adult; D000818:Animals; D002681:China / Q000453:epidemiology; D005260:Female; D006801:Humans; D064766:Influenza A Virus, H7N9 Subtype / Q000302:isolation & purification* / Q000502:physiology; D005585:Influenza in Birds / Q000453:epidemiology / Q000635:transmission* / Q000821:virology; D007251:Influenza, Human / Q000453:epidemiology / Q000635:transmission* / Q000821:virology; D008297:Male; D008875:Middle Aged; D011200:Poultry; D055815:Young Adult
keywords: Epidemiology; Family clusters; H7N9 influenza; Live bird markets; Secondary cases