Abstract
To study human-to-human transmissibility of the avian influenza A (H7N9) virus in China, household contact information was collected for 125 index cases during the spring wave (February to May 2013), and for 187 index cases during the winter wave (October 2013 to March 2014). Using a statistical model, we found evidence for human-to-human transmission, but such transmission is not sustainable. Under plausible assumptions about the natural history of disease and the relative transmission frequencies in settings other than household, we estimate the household secondary attack rate (SAR) among humans to be 1.4% (95% CI: 0.8 to 2.3), and the basic reproductive number R0 to be 0.08 (95% CI: 0.05 to 0.13). The estimates range from 1.3% to 2.2% for SAR and from 0.07 to 0.12 for R0 with reasonable changes in the assumptions. There was no significant change in the human-to-human transmissibility of the virus between the two waves, although a minor increase was observed in the winter wave. No sex or age difference in the risk of infection from a human source was found. Human-to-human transmissibility of H7N9 continues to be limited, but it needs to be closely monitored for potential increase via genetic reassortment or mutation.
Highlights
Influenza A (H7N9) was first detected among humans in eastern China in February 2013 and, as of 7 December 2014, had caused 453 laboratory-confirmed clinical human infections with 178 deaths in China, according to the Chinese Center for Disease Control and Prevention (Chinese CDC)
While the ongoing sporadic reporting of cases implies the H7N9 virus has not yet reached the stage of efficient human-to-human transmission, an animal model has shown another H7 virus was able to transmit among co-housed ferrets without much loss of virulence, with a few mutations obtained after 10 serial passages [13]
There was an asymptomatic infection identified in one household in the first wave, but this person was considered a non-case in our analysis; i.e., we only focus on laboratory-confirmed infections with clinical symptoms, because (i) it is not clear whether asymptomatic hosts can transmit the virus or not, and (ii) probably not all asymptomatic infections were detected
Summary
Influenza A (H7N9) was first detected among humans in eastern China in February 2013 and, as of 7 December 2014, had caused 453 laboratory-confirmed clinical human infections with 178 deaths in China, according to the Chinese Center for Disease Control and Prevention (Chinese CDC). The virus is a reassortant of avian H7, N9 and H9N2 strains [1] with evidence of the capacity to bind to mammalian cells [2,3] and limited airborne transmission in animal models [4,5]. The virus is not pathogenic in birds, but highly pathogenic and virulent in humans [6,7,8,9]. While the ongoing sporadic reporting of cases (per communication with the Chinese CDC) implies the H7N9 virus has not yet reached the stage of efficient human-to-human transmission, an animal model has shown another H7 virus was able to transmit among co-housed ferrets without much loss of virulence, with a few mutations obtained after 10 serial passages [13]. It is highly relevant to assess the risks of human-to-human transmission using available data
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