Pandemic viruses hard to make, US scientists say

WASHINGTON, Aug 1: Scientists who spliced a common flu virus with the H5N1 bird flu virus said they failed to make a strain that could cause a pandemic.

The team at the US Centers for Disease Control and Prevention in Atlanta is working in high-security labs and says their work poses no risk to the public.

''This research study was undertaken in order to better understand the genetic changes that are needed for an H5N1 virus to acquire the genetic changes needed for better transmission,'' Dr Jackie Katz of the CDC's Influenza Branch told reporters in a telephone briefing yesterday.

One way viruses can evolve into epidemic and pandemic forms is by mixing their genetic material with other viruses. Flu pandemics that killed millions in 1957 and 1968 did this.

So Katz's team tried mixing a sample of H5N1 virus taken in 1997 with genes from an H3N2 flu virus -- which caused the 1968 and 1957 pandemics and that still sickens people every year.

They tested it in ferrets, which catch flu in a way very similar to people. Some of the mixtures made the ferrets sick, but infected ferrets put into a cage next to healthy ferrets did not pass on the virus.

CDC Director Julie Gerberding said the findings were not reassuring.

''These data do not mean that H5N1 cannot convert to be transmitted from person to person. They mean that it is not a simple process,'' she told reporters.

There are more than 50 possible combinations of the viruses, according to Gerberding, and Katz said her team could have created a dangerous strain of flu.

''Of course the possibility was there that we would generate a strain that had the property of transmission,'' Katz said. ''That's what the question of the study was. But the study was conducted using the most stringent safety precautions that protected both the workers and the public.''

SPREAD WIDELY The H5N1 virus remains largely a bird disease, but it has spread across much of the world in the past year. It has killed 134 people out of 232 known to be infected. Some infected people have infected other people with H5N1 but only in rare instances, among family members.

''If H5N1 viruses acquire the ability to undergo efficient and sustained transmission among humans, a pandemic would be inevitable,'' the researchers wrote in their report, published in the Proceedings of the National Academy of Sciences.

It can do this by acquiring steady mutations, or by reassorting with another flu virus, which can happen, for instance, if a person becomes infected with two different strains at the same time.

''We also know that H5N1 continues to undergo microevolution, a series of genetic changes that are subtle,'' Gerberding said.

Katz's team tried putting H3N2 surface proteins, which match human cells, onto the H5N1 virus. They also stuck H5N1 genes into an H3N2 virus.

None were passed between the ferrets. They also observed that the ferrets sneezed when they were infected with a normal H3N2 virus, but not when they were infected with the genetically engineered strain.

That suggests the ability of the virus to make its victims sneeze and cough is key to transmitting it.

Katz said her team will continue to work with the viruses.

REUTERS