Developing a vaccine for the H7N9 strain of bird flu that has killed 10 people in eastern China could take "many months", US public health experts have said.

Chinese authorities had confirmed 38 human cases of H7N9 avian influenza as of Thursday evening after announcing nearly two weeks ago that they had found the strain in humans for the first time.

Earlier vaccines targeting similar viruses had produced poor results, two members of the influenza division of the US Centers for Disease Control and Prevention (CDC) wrote in the New England Journal of Medicine.

"Even if new vaccine manufacturing technologies... are utilised, the process from vaccine development to availability will probably take many months," said Timothy Uyeki and Nancy Cox.

Although worldwide efforts to develop a vaccine had already started based on early cases, clinical trials would also be needed, said the article posted on the journal's website on Thursday.

The Chinese government said Wednesday that it expected to have a vaccine ready within seven months, state media reported.

China's Ministry of Science and Technology and the health ministry were jointly working on the vaccine, the Xinhua news agency said.

China has also developed a test for H7N9, distributing kits to hospitals and other monitoring sites, Xinhua said.

The journal article praised China's speed in identifying the virus, though it called for enhanced surveillance of people with respiratory illness and monitoring of the contacts of infected patients.

Chinese authorities say they do not know how the virus is spreading but they have so far ruled out human-to-human transmission, a conclusion echoed by the World Health Organization.

Animal health agency: Bird flu poses 'exceptional situation'
Paris (AFP) April 11, 2013 - The World Organisation for Animal Health (OIE) on Thursday said H7N9 bird flu posed an "exceptional situation" as the outbreak among Chinese poultry claimed a 10th human victim.

"Based on the information currently available we are facing a rather exceptional situation," said OIE chief Bernard Vallat, explaining that the virus, while dangerous to humans, was hard to detect in the host, which is farm birds.

"We are dealing with an influenza virus of very low pathogenicity for poultry which has the potential to cause severe disease when it infects humans," Vallat said in a statement.

The Paris-based OIE said that, based on reports sent to it by the Chinese veterinary authorities, infected birds "do not show any visible signs of disease, making it very difficult to detect this virus in poultry."

The OIE is the world's monitor for the health of farm animals traded across borders.

Past food crises it has handled include the mad-cow scare and the H5N1 bird flu, a different strain of avian influenza that first surfaced in Hong Kong in 1997.

On Thursday, China said the death toll from H7N9 reached 10 out of 38 confirmed human cases, with another fatality in Shanghai.

The virus is believed to spread to humans from birds, triggering the mass culling of poultry in several Chinese cities.

The fear is that the virus will mutate, making it transmissible from human to human.

The OIE said it had been informed of eight outbreaks of H7N9 in pigeons and chickens at Chinese markets, "all located in Shanghai and neighbouring provinces."

The possible reservoir for the virus in nature is being probed by the China Animal Disease Control Centre and China's animal health service, including a world-standard laboratory, the Harbin Veterinary Research Institute.

"At the international level, the Food and Agriculture Organisation of the United Nations (FAO), the World Organisation for Animal Health (OIE) and the World Health Organisation (WHO) are working together to support China's efforts to manage this new and evolving situation," the OIE statement added.