The virus has sickened at least 43 people in eastern China. Most of the infections have involved severe illness and 11 of the cases have died.
And late Friday — Saturday in China — health authorities in Beijing revealed the capital has recorded its first H7N9 case, in a seven year old girl whose parents are in the live poultry trade. That marks a big and worrisome geographic jump for a virus which till now had only been seen in three provinces around Shanghai.
People are getting sick, but for the most part it's not apparent how they were infected. That's because in poultry the virus is of low pathogenicity — "low path" in the vernacular of flu. It is not currently causing evident disease in birds, making it the viral equivalent of a stealth bomber.
Though its genetic blueprint makes clear it is a bird flu virus, H7N9 carries a number of mutations that indicate it is partially adapted to infecting mammals — which may help explain the rapidly accumulating human cases.
To date it doesn't appear as if the virus is spreading person to person, which is perhaps the best feature of this virus. But two weeks after China announced it had found people infected with a new flu, concern among those in the influenza research world remains high.
"I think we are genuinely in new territory here in which the situation of having something that is low path in birds (yet) appears to be so pathogenic in people," Dr. Keiji Fukuda, the WHO's assistant director-general for health security and environment, said in an interview.
"And then to have those genetic changes ... I simply don't know what that combination is going to lead to."
The situation is quintessential flu. The eight-gene RNA viruses are notorious for throwing knuckle balls, catching even those who study them for years off guard.
"Almost everything you can imagine is possible. And then what's likely to happen are the things which you can't imagine," Fukuda, who spent years as an influenza epidemiologist at the U.S. Centers for Disease Control before joining the WHO, said of the virus he has studied for so long.
At this point, few in the flu world would place strong bets on what the history books will say about this outbreak. So in the face of the uncertainty, the WHO is laying the groundwork for action, if action is needed.
As of Thursday, all the laboratories in the WHO's flu network have received copies of the H7N9 virus — a development that allows them to dive into research on the virus. Fukuda praised Chinese authorities for sharing the virus so quickly and broadly.
One of the recipients, the CDC, said Friday it will use the virus to develop a seed strain for an H7N9 vaccine, evaluate whether diagnostic tests that have been developed actually detect the virus, confirm whether the viruses are susceptible to the two main flu drugs, oseltamivir (Tamiflu) and zanamivir (Relenza), and try to develop a blood test that could detect H7N9 antibodies in people who have been infected.
That test would also help laboratories figure out if infections with other flu viruses might give people some cross-protection against this new one.
Fukuda said the WHO has been reaching out to doctors in China, sharing information about treating severe influenza infections. Some of that knowledge derives from nearly a decade of sporadic human cases of H5N1 bird flu, which like H7N9 can cause severe disease. And some of the knowledge was garnered during the 2003 SARS outbreak.
One piece of guidance the agency is trying to share involves discouraging the use of high-dose corticosteroids. The powerful drugs were used in some countries during SARS, and were tried with some H5N1 cases. Not only did the drugs not help, but they were also counterproductive.
Fukuda said the WHO is also working on "what ifs" at this point, figuring out what needs to be in place if an H7N9 vaccine is needed, if the WHO has to send antiviral drugs to help with the response, or if it needs to heighten the warnings it is giving to the world about H7N9's pandemic potential.
"There are still a lot of fundamental questions open, but when you put it all together, this is a quite serious signal for us," Fukuda said. "And there's nothing which has lessened the seriousness of that signal over the last few weeks."
— With files from The Associated Press
Also on HuffPost