updated 4:19 AM EDT, Wed April 24, 2013Hong Kong (CNN) -- As the death toll from China's bird flu outbreak rose to 22 with news of another victim in eastern Zhejiang Province, the World Health Organization warned the H7N9 virus was one of the most lethal that doctors and medical investigators had faced in recent years.
"This is an unusually dangerous virus for humans," Keiji Fukuda, WHO's assistant director-general for health, security and the environment told a news conference in Beijing Wednesday.
"We think this virus is more easily transmitted from poultry to humans than H5N1," he added, referring to the bird flu outbreak between 2004 and 2007 that claimed 332 lives.
"This is definitely one of the most lethal influenza viruses that we have seen so far."
As investigations continue into the possible sources of infection, Fukuda warned that authorities were still struggling to understand the virus. The WHO said China must brace for continued infections.
"I want to give you a caveat, or give you a little bit of context. We really are at the beginning of our understanding of this virus," Fukuda said. "(The situation remains) complex, difficult and it is evolving."
So far there is no evidence of sustained human-to-human transmission, the authorities say.
"We do want to note, however, that if limited person-to-person transmission is demonstrated in the future, this really will not be surprising," Fukuda warned, adding that it was critical to remain vigilant, monitoring the virus's spread and mutation.
"We are not sure that the clusters were caused by common exposure to a source of the virus or were due to limited person-to-person transmission," he said. "Moreover we have not seen sustained person-to-person transmission."
While some elements of the outbreak have baffled investigators -- specifically why the virus tends to target an elderly demographic and the fact that it is asymptomatic or mild in some cases and lethal in others -- authorities have claimed some significant victories in the fight against a pandemic.
Anne Kelso, the director of a WHO-collaborating research center, said researchers had seen a "dramatic slowdown" in human cases in Shanghai after the city's live poultry market was shut on April 6. Describing the finding as "very encouraging," she said evidence suggests the closure of live poultry markets is an effective way to stop the spread of the virus.
This is an unusually dangerous virus for humans
Keiji Fukuda
Keiji Fukuda
The joint inspection team from China's National Health and Family Planning Commission and the World Health Organization also found that, so far, no migratory birds have tested positive for the virus, taking another worrying route of transmission out of the equation.
It said the H7N9 virus is only being found in chickens, ducks and pigeons at live poultry markets.
WHO officials said there are already efforts underway in other countries to develop a vaccine after Chinese officials admitted international help would be needed with this.
Meanwhile, the National Health and Family Planning Commission said in its daily update on H7N9 cases that a total of 108 H7N9 cases have been reported in China, including 22 deaths. Most cases have been confined to Shanghai and neighboring provinces in eastern China. http://www.cnn.com/2013/04/24/world/asia/china-birdflu/index.html