Only 10 of the 77 people who've been infected with the latest bird flu strain in China had contact with poultry, the World Health Organization said Wednesday.

Whether the H7N9 influenza A strain might be spreading by vectors other than poultry, however, the Chinese Center for Disease Control and Prevention declined to say. Sixteen people have died from the disease thus far and the number of infections may soon rise, they said.

As the U.S. Centers for Disease Control and Prevention continues to rush develop of a vaccine for the strain, in addition to a diagnostic blood test, an international team of epidemiologists flew to China.

Although WHO said no evidence exists that the disease spreads through human transmission, "We can't rule this out," Wendy Barclay, a flu expert at Imperial College London, told Reuters. "The incubation time might be quite long so visiting a market even 14 days before might have resulted in infection."

Nearly 40 percent of those infected with the strain had no contact with poultry, said Zeng Guang, China's chief scientist in charge of epidemiology.

WHO spokesman Gregory Hartl also said "there are people who have had no history of contact with poultry" but said he didn't know the exact percentage among the infections. "This is one of the puzzles still [to] be solved and therefore argues for a wide investigation net," he said. There is "no evidence of sustained human-to-human transmission."

The disease might possible be spread by another animal source or dust at wet markets, he said.

Researchers believe the influenza strain is a "triple-reassortant" virus, an admixture of genes from thre other strains found in birds in Asia. While two of the strains come from poultry, another might have come from a brambling, a small wild bird. This week, Chinese authorities reported three new influenza outbreaks in birds to the World Animal Health Organization, for a total of 11 outbreak areas in China.

Authorities there have slaughtered thousands of birds and closed live poultry markets in Shanghai and Beijing with industry losses at more than $1.6 billion. On Sunday, China said the virus had spread outside the Yangtze River delta region in eastern China, confirming cases in Beijing and the central province Henan.

With no licensed vaccine and so many unknowns about the infectious disease, Chinese consumers are turning to the traditional medicine in fear of a pandemic, buying large quantities of the herb ban lan gen, also known as blue root.

"Chinese people associate ban lan gen with anti-virus," said Shen Jiangang, assistant director for research at the Chinese University of Hong Kong's School of Chinese Medicine. "So when they hear about bird flu, they immediately think it might be effective to protect themselves, although there is no experimental evidence."

Shares of Beijing Tongrentang, the product manufacturer, closed for an increase of 1.9 percent to a two-week high of 22.20 yuan in Shanghai. Surgical face masks are also selling well.