"Prototype viruses for vaccine production are being prepared by laboratories in the WHO Global Influenza Network," which would then be used as seed stock for vaccine production, the WHO regional office here said.

It said "a prototype virus could be made available to vaccine manufacturing companies within about four weeks," but that it would take several steps before the vaccine could be ready for use in humans.

The H5N1 strain of the flu has killed five people in Vietnam and has caused massive poultry culls and trade disruption with other countries taking steps to protect their human and bird populations from the epidemic.

Laboratories in Hong Kong and Japan have isolated the virus from specimens obtained from two of the fatal cases in Vietnam, it said in a written statement.

The virus for use in influenza vaccines is grown in chicken eggs. However, because H5N1 is so deadly in chicken embryos, a new technique known as "reverse genetics" is required to prepare the prototype virus for vaccine production.

Reverse genetics merges selected genetic information of the virus taken from actual cases with a laboratory virus. The resulting virus is recognized by the human immune system, and causes a protective immune response, but no disease.

The virus can also be genetically modified so that it is no longer lethal to chicken embryos. Reverse genetics produces a prototype virus with predictable growth during vaccine production, it said.

The prototype virus is then used by manufacturers to produce sample vaccines for clinical testing.

"WHO will offer support in the coordination of these clinical trials, which are needed to determine the amount of vaccine and number of doses required to confer protection, also in different age groups," it added.

The virus strain involved in the current epidemic has not been spread from person to person.

"Historically, influenza pandemics have spread rapidly around the world, causing high mortality and affecting all age groups," the WHO said. The most severe pandemic in the previous century, in 1918-1919, killed an estimated 50 million people.

The bird flu epidemics are the first in Japan since 1925, and the first ever documented in Vietnam and South Korea, the WHO said.

Candidate vaccines were developed last year by network laboratories in London as well as Memphis, Tennessee, for protection against the H5N1 virus strain that caused two cases and one death in Hong Kong in February 2003.

"If the virus isolated from the fatal cases in Vietnam proves sufficiently similar to the 2003 H5N1 strain in Hong Kong, the existing candidate vaccines could expedite the availability of a new vaccine," the WHO said.

TERRA.WIRE