By William Hudson, CNN
(CNN) — The H7N9 bird flu virus, first identified in humans earlier this year, kills about 36% of infected people admitted to hospitals in China, according to a new report published Sunday in the British medical journal The Lancet.
Far more difficult to estimate, according to the study, is how many die in the general population after becoming infected, as the most severe cases are also more likely to lead to hospitalization.
That estimate — a 0.16% to 2.8% overall fatality rate for those showing symptoms of infection — suggests that the H7N9 virus is less deadly than the H5N1 Bird Flu first appearing in 2003, and more deadly than the 2009 H1N1 swine flu pandemic.
Others infected with the virus may never show symptoms.
“There’s almost always a large portion of asymptomatic (flu virus) cases, and cases where infected people don’t seek treatment,” says Dr. Gabriel Leung, one of the study’s authors and head of the University of Hong Kong’s School of Public Health.
The rate of infections seen earlier this year has nearly stopped entirely at 132 total confirmed cases, almost all in China and one in Taiwan, according to the World Health Organization and the study. Thirty-seven people have died. Only one case has appeared since May 8 – a 6-year-old boy in Beijing.
Still, public health officials warn that number could significantly rise again during the autumn flu season.
In contrast to H1N1 swine flu, which is now part of the seasonal flu vaccine, H7N9 and H5N1 flu viruses most commonly occur after exposure to infected poultry, and do not appear to spread easily from person-to-person.
In April, the World Health Organization warned the H7N9 virus was “one of the most lethal” that doctors and medical investigators had faced in recent years.
“The preemptive closure of live poultry markets made a difference,” says Leung. “They were closed within weeks of the initial cluster of cases in and around the Yangtze delta. If those live poultry markets are reopened, we may expect to see cases again.”
Symptoms of H7N9 flu infection start with high fever and cough, and in severe cases progress to pneumonia and multi-organ failure, according to the Centers for Disease Control and Prevention.
The risk to people in the United States is low, and no cases have been detected in the country, the CDC says.