No Change in Human-to-Human H7N9 Transmission Risk Over Time

Lei Zhou, MD

Despite the increase in the number of human infections with avian influenza A (H7N9) virus during the fifth epidemic in China, a new study suggests no change in the risk of transmission of the virus among humans over time.

Lei Zhou, MD, from the Public Health Emergency Center of the Chinese Center for Disease Control and Prevention, Beijing, China, and Enfu Chen, MPH, from the Zhejiang Center for Disease Control and Prevention, Hangzhou, China, and colleagues published the results of their study in the February 2018 issue of Emerging Infectious Diseases, the monthly peer-reviewed public health journal of the US Centers for Disease Control and Prevention (CDC).

In an interview with Contagion^®, Dr. Zhou explained that the fifth epidemic wave of human infections with avian influenza A (H7N9) virus differed from previous epidemics by a large increase in human infections that occurred over a wide geographic area of China, including the emergence of human infections with highly pathogenic avian influenza A (H7N9) virus.

“Risk assessments by the CDC and the World Health Organization (WHO) both concluded that avian influenza A (H7N9) virus poses the greatest threat of an influenza pandemic of all novel influenza A viruses,” she said.

“Efficient and sustained human-to-human transmission is one of the key features of a potential pandemic influenza A virus.”

Given the surge in human infections with influenza A (H7N9) virus in the fifth epidemic, Dr. Zhou and colleagues, therefore, wanted to determine if the risk of human-to-human transmission of the virus has increased over time.

The researchers examined data from patients with laboratory-confirmed A (H7N9) infection that were reported to the China CDC between February 1, 2013, and June 30, 2017. They analyzed characteristics of 40 clusters of case-patients during 5 epidemics within this period of time.

The researchers identified 84 infections comprising 40 clusters among 1524 influenza A (H7N9) case-patients. According to the authors, 14 clusters occurred in the fifth epidemic, compared with 4 to 11 clusters in earlier epidemics. The 14 clusters in the fifth epidemic comprised 29 human infections, they added, while previous epidemics each had clusters of 8 to 23 human infections.

Among the 40 clusters for all 5 epidemics, they classified 14 (35%) as probable and 26 (65%) as possible human-to-human transmission. “The proportion of clusters with probable human-to-human transmission only did not differ significantly by epidemic,” the authors note.

Dr. Zhou stressed that “the similarity in numbers and sizes of clusters and proportions of clusters with probable human-to-human transmission during 2013—2017 suggest no change in human-to-human H7N9 virus transmission risk over time.”

Therefore, the large increase in human infections during the fifth epidemic probably reflects an increase in sporadic poultry-to-human H7N9 virus transmission over a wide geographic area in China, she said.

These findings will be helpful for public health practitioners to assess the potential pandemic risk caused by the H7N9 virus, added Dr. Zhou.

“We also found that limited, non-sustained human-to-human transmission of A (H7N9) virus has occurred in health care settings, highlighting the importance of adhering to recommended infection prevention and control measures to prevent nosocomial A (H7N9) virus transmission,” said Dr. Zhou.

She also strongly recommended ongoing assessment of the epidemiology of human infections with influenza A (H7N9) virus, to identify any increase in human-to-human transmission, which could suggest an increasing pandemic threat.

The investigators will continue to study the epidemiology of human infections with influenza A (H7N9) virus, including a severity assessment, the spectrum of illness in infected persons, and transmission dynamics.

“We expect a sixth epidemic of human infections with influenza A (H7N9) virus this coming winter and need to be prepared to assess any changes in the epidemiology that could signal an increasing pandemic threat,” Dr. Zhou concluded.

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.