Scientists in Hong Kong and the United States have developed an experimental H5N1 bird flu vaccine for people by piggybacking it on the well-tested and highly successful smallpox vaccine.

Initial tests on mice showed the vaccine to be highly effective, they told a news conference in Hong Kong yesterday.

"It produced a lot of (H5N1) antibodies and the speed of antibody response was far higher with this strategy than the Sanofi one," said Malik Peiris, a microbiologist and bird flu expert at the University of Hong Kong.

Peiris was referring to Sanofi-Aventis's H5N1 bird flu vaccine for humans, which has been approved for use in the United States.

In an article published in the Journal of Immunology, the experts from the special administrative region and the US National Institutes of Health described how they inserted five key components of the H5N1 virus into the smallpox vaccine.

"We put in many other proteins into that vaccine; we are using it like a carrier, if you like, a piggyback," Peiris said.

The vaccine uses a Vietnamese strain of the H5N1 virus and appeared to be broadly protective. Mice which were inoculated with it successfully fought off an Indonesian strain of H5N1, according to the scientists.

Since 2003, the H5N1 avian influenza virus has infected 408 people across the world and killed 254 of them.

It has killed or forced the culling of about 300 million birds as it spread to about 60 countries in Asia, the Middle East, Europe and Africa.

While H5N1 rarely infects people, experts fear it could mutate into a form that people could easily pass to one another, sparking a pandemic that could kill tens of millions.

Smallpox was eradicated worldwide in 1979, and the experts are hoping that their novel H5N1 vaccine can ride on the various advantages of the smallpox vaccine.

The smallpox vaccine is very cheap, has a long shelf-life of several years and does not require highly sophisticated laboratories, making it easier for poorer countries to produce.

However, it will take at least a few more years before the vaccine is ready for the market. It must be tested next in ferrets, then monkeys, before human clinical trials can be carried out.

(Xinhua News Agency March 2, 2009)