Two of three promised samples of bird flu in China have arrived in the U.S., the WHO said. The samples are the first sent by Beijing in a year.
The sample updates of the H5N1 virus from China's Health Ministry are awaiting customs clearance, said Joanna Brent, a spokeswoman for WHO's Beijing office.
They include specimens from a 2006 case in Xinjiang in China's far west, and a case in the southern province of Fujian in 2007, Brent said.
But a sample from a 24-year-old soldier who died in 2003 in Beijing was not part of the batch, she said. The Health Ministry in April had promised one from that case, which was disclosed last year after new tests determined he had succumbed to the disease.
The ministry "says the procedures for sharing the Beijing 2003 samples involve the military and are extremely complex," Brent said.
The Chinese military, a power unto itself in China, and is usually secretive about its operations.
China has already sent six bird flu virus samples from humans to WHO's designated laboratories.
Two of them were dispatched in December 2005 and the others in May 2006, state media have reported. Since then, five new human cases have been reported in China.
WHO does not mandate sharing virus samples, although they are needed to produce diagnostic tools and vaccines. The lack of cooperation, experts say, could slow efforts to track diseases and develop vaccines and other strategies to deal with them.
According to WHO, bird flu has killed at least 186 people since H5N1 started ravaging poultry flocks in late 2003. Experts have warned that if outbreaks in birds are not controlled, the virus may mutate into a form more easily passed between people, potentially resulting in millions of deaths worldwide.
China has in the past been slow to share animal samples, and international health experts have repeatedly complained about Chinese foot-dragging in cooperating on investigating emerging diseases like bird flu and pneumonia from SARS, or secure acute respiratory syndrome.
Beijing has since vowed an aggressive fight against H5N1, launching a massive vaccination program and reporting improved surveillance systems.
At the World Health Assembly earlier this week, U.N. health officials welcomed a tentative agreement to share bird flu virus samples.
The preliminary deal was reached after months of debate and strong opposition from developing countries, which argue that the existing system of unconditional sample sharing puts poorer nations at a disadvantage because they are unable to afford commercial vaccines.
Some critics fear the situation could potentially be even more unfair if the vaccines are created from virus samples the poor countries supplied to WHO.
The agreement's implementation is voluntary, and exceptions are possible.