H7N9 Cases May Be Double Known Figure, Hong Kong Researchers Say

H7N9 bird flu may have infected twice as many people as the 103 cases reported, an analysis by researchers at the University of Hong Kong showed.

There may be 90 to 120 ill adults who haven’t been detected because their infections are mild, Benjamin Cowling, associate professor at the university’s public health research center, said today. The researchers’ analysis suggests risk of serious illness from the virus rises substantially with age, with more than half of reported cases age 60 or older, he said.

Flu specialists including those from the World Health Organization are investigating how people are catching the H7N9 virus, with no evidence yet of sustained human-to-human transmission. Disease trackers haven’t been able to figure out why another deadly bird flu strain known as H5N1 afflicts mostly younger people in their 20s and 30s, while H7N9 mainly targets the elderly.

“One thing that is very striking is the age distribution of the cases,” Cowling said at a briefing at the university’s medical school today. “They’re very different from the confirmed infections of H5N1.”

The estimates were derived from a model using publicly available data, said Gabriel Leung, director of the university’s public health center. Further studies will be conducted, the researchers said.

The WHO is still assessing the H7N9 outbreak and investigations haven’t proved any transmission from person to person, Keiji Fukuda, the Geneva-based agency’s assistant director-general for health security and environment, said at a briefing in Shanghai today.

Twenty people have died among the 103 H7N9 cases reported by China. A majority of the patients are in eastern China, with Shanghai and Zhejiang province having the most cases among regions there.