An international team of experts concluded an investigative mission to China today with both sobering and encouraging findings about H7N9, a novel avian influenza virus recently found for the first time in humans.
"This is an unusually dangerous virus for humans," said Keiji Fukuda, assistant director-general for health security of the World Health Organization (WHO), at a press conference in Beijing this morning. From what is known so far, he added, H7N9 "is more easily transmissible from poultry to humans than H5N1," the avian influenza virus that has circulated in poultry in Asia for more than a decade, occasionally causing human fatalities.
The team also reported that the available evidence points to live bird markets as being the most likely pathway for the virus from poultry to humans. Positive samples have been retrieved from poultry and from contaminated surfaces at the markets. Nancy Cox, a flu expert at the Centers for Disease Control and Prevention in Atlanta, cautioned that while it is still early, "we can now understand that the likely source of infection is poultry—that the virus originates from poultry."
Shanghai closed its live poultry markets on 6 April, shortly after the market link was suspected. "Almost immediately there was a decline in the number of new cases," said Anne Kelso, director of the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne, Australia. "This is a very encouraging outcome so far," she added, calling the decision to close markets "very quick and appropriate."
But Kelso also stressed that there is a need for continuing vigilance. "It's going to be very important to watch over the next days, weeks, even months, what happens as a result of a shutdown of the live markets. It's possible other routes of infection will be found that we don't know about yet."
So far, researchers have found very little evidence of the virus circulating on farms, noted virologist Malik Peiris of the University of Hong Kong, but "you don't need many infected farms to have the virus amplifying in the live poultry markets and causing a threat to humans," he said.
As of 23 April, H7N9 had infected 108 people, causing 22 deaths, according to WHO. That total does not include the first case outside mainland China, confirmed by the Centers for Disease Control in Taipei  late this afternoon local time. The patient is a Taiwanese citizen who fell ill 3 days after returning to Taiwan from Jiangsu province, the site of several confirmed H7N9 cases. Like many H7N9 victims, he could not recall any contact with poultry.
The first known human H7N9 cases surfaced in Shanghai in late February, though the virus was not identified until later. The joint China-WHO mission was called together to assess the situation and recommend how to control the outbreak. The team warned that the world is still in the very early stages of gauging the severity of H7N9. "We don't know the extent of the public health risk of the current virus and infection and how we can deal with such risks," said Liang Wannian, an official with China's National Health and Family Planning Commission who co-led the team with Fukuda. He added that understanding the epidemiology of the disease in both humans and animals requires the efforts of "the best experts of the world."
With one in five known infections resulting in death so far, "this is definitely one of the most lethal influenza viruses that we have seen so far," Fukuda said. But he also cautioned that only the most serious infections may have been caught until now; there may be many milder cases, or asymptomatic ones, like that of a 4-year-old boy identified in Beijing last week. "You could see [that case] in an encouraging way, because it could be that there are a larger number of individuals with very mild or asymptomatic ailments," said Angus Nicoll, head of the influenza program at the European Centre for Disease Prevention and Control in Solna, Sweden. This would mean the fatality rate is not as high as it now appears to be.
Fukuda, who profusely praised Chinese authorities for their handling of the crisis in his opening statement , said that the few family clusters identified so far in China could be due either to common exposure or to "limited person-to-person transmission." But he emphasized that "no sustained person-to person-transmission has been found."
Now that the mission is over, China will continue its surveillance efforts for both animal and human health, Liang said: The immediate priority is to "clearly identify the source of the virus."