Nosocomial transmission of avian influenza A (H7N9) virus in China: epidemiological investigation

Dr. Fang and colleague reported evidences on the nosocomial transmission of H7N9 influenza virus between two hospitalized patients [1]. Recently, nosocomial patient-to-doctor transmission of H7N9 influenza virus had also been documented [2]. It is time to re-assess the human-to-human transmission potential of this novel virus.

The human-to-human transmissibility of a virus is measured by its basic reproductive number (R0) among human, defined as the average number of the secondary cases generated from a single infectious case introduced into an all-susceptible population. If the R0 is less than 1, then the human-to-human transmission will occur only sporadically and is not sustainable. On the other hand, if the R0 is larger than 1, then the virus will cause hospital-associated large outbreaks (the probability increases with a higher R0) unless effective interventions have been promptly implemented.

Using China nationwide surveillance data, we estimated the R0 of H7N9 virus is only 0.07 by De Serres’ proportion of cases method [3,4]. Other researchers, using various modeling methods, also estimated the R0 of H7N9 in the range of 0.03-0.4 [5-9]. The less-than-1 R0 indicates that H7N9 virus is unlikely to cause hospital-associated large outbreaks. Nevertheless, nosocomial transmission still can sporadically occur in the absence of infection control measures. To ensure the safety of patients and staff, it is imperative for health care workers to adhere to the infection control guidelines issued by Centers for Diseases Control and Prevention when providing care for patients with suspected H7N9 infections [10].

Chi-Tai Fang, MD, PhD
National Taiwan University Hospital, Taipei, Taiwan
fangct@ntu.edu.tw

Zhifei Liu, MPH
School of Public Health, Southeast University, Nanjing, mainland China

Reference
1. Fang CF, Ma MJ, Zhan BD, et al. Nosocomial transmission of avian influenza A (H7N9) virus in China: epidemiological investigation. Bmj 2015;351:h5765.
2. Farooqui A, Liu W, Zeng T, et al. Probable hospital cluster of H7N9 influenza infection. N Engl J Med 2016;374:596-98.
3. Liu Z, Fang CT. A modeling study of human infections with avian influenza A H7N9 virus in mainland China. Int J Infect Dis 2015;41:73-8.
4. De Serres G, Gay NJ, Farrington CP. Epidemiology of transmissible diseases after elimination. Am J Epidemiol 2000;151:1039-48; discussion 49-52.
5. Tanner WD, Toth DJ, Gundlapalli AV. The pandemic potential of avian influenza A(H7N9) virus: a review. Epidemiol Infect 2015;143:3359-74.
6. Qin Y, Horby PW, Tsang TK, et al. Differences in the epidemiology of human cases of avian influenza A(H7N9) and A(H5N1) viruses infection. Clin Infect Dis 2015;61:563-71.
7. Chong KC, Wang X, Liu S, et al. Interpreting the transmissibility of the avian influenza A(H7N9) infection from 2013 to 2015 in Zhejiang Province, China. Epidemiol Infect 2015:1-8.
8. Yang Y, Zhang Y, Fang L, et al. Household transmissibility of avian influenza A (H7N9) virus, China, February to May 2013 and October 2013 to March 2014. Euro Surveill 2015;20:21056.
9. Kucharski A, Mills H, Pinsent A, et al. Distinguishing between reservoir exposure and human-to-human transmission for emerging pathogens using case onset data. PLoS Currents 2014;6.
10. Centers for Disease Control and Prevention. Interim guidance for infection control within healthcare settings when caring for confirmed cases, probable cases, and cases under investigation for infection with novel influenza A viruses associated with severe disease. 2014. http://www.cdc.gov/flu/avianflu/novel-flu-infection-control.htm.