Scientists fear the avian flu that has killed 46 people in Asia could be the strain that will cause the next global pandemic but said more evidence is needed about how infectious it is in humans.

“I think pandemic flu is knocking on the door,” said Professor Albert Osterhaus, a leading European virologist at Erasmus University Hospital in Rotterdam.

As bird flu experts meet in Asia to devise plans to combat the H5N1 virus, scientists said they lacked knowledge about whether the strain that has led to the slaughter of tens of millions of birds has the pandemic potential of the 1918 Spanish flu that killed between 20 million and 40 million people.

To become a pandemic strain, H5N1 would have to adapt sufficiently on its own, or mix its genetic material with a human virus to become highly infectious in humans who have no protection against it.

“We don’t know whether the virus that is currently circulating among poultry in southeast Asia, the H5N1, will eventually be able to reassort its genetic material with a human influenza virus. That is the key question,” Osterhaus told Reuters in an interview.

So far the H5N1 strain has shown no evidence that it has become highly infectious in humans. Scientists also do not know how many people may have been exposed to it.

But Laurence Tiley, a molecular virologist at Cambridge University in England, said the H5N1 strain is lethal.

“It is the most likely candidate for adapting and becoming a pandemic strain because we are not going to be able to get rid of it easily,” he said.

WHEN NOT IF

Health experts are worried about H5N1 because like previous pandemic strains it carries a new combination of two molecules - hemagglutinin (H) and neuraminidase (N).

“The hemagglutinin and neuraminidase are the surface proteins of the virus. They are the most important proteins as far as the host immune system is concerned,” Tiley said.

H1N1, H2N2 and H3N2 were strains of other flu pandemics.

If a person is exposed to H5N1 and is also co-infected with a human flu, components of both could produce a virus that could be transmissible from person to person and capable of spreading widely and very rapidly across the world.

“The whole population would be completely naive and vulnerable to it. That’s why the previous pandemics have coincided with the acquisition of a new H or N surface protein on the new viruses,” said Tiley.

“We know that flu has done this sort of thing in the past.”

The H5N1 has killed a high percentage of people who have been diagnosed with it. But so far it has not shown any clear evidence of an ability to spread easily in humans.

“This particular strain is very lethal. But at the moment it is very poorly contagious,” said Tiley.

The extent of the population’s exposure to the virus is also important because it gives scientists an idea of how much opportunity the virus has had to evolve.

“There are figures (numbers) of people who have become ill or died but we do not know how many people have been infected,” said Osterhaus.

“Blood tests are needed in the population to see the extent to which it has spread. It should be given high priority,” he added.

If the H5N1 strain does not have pandemic potential, Osterhaus believes all the factors that have led to previous influenza pandemics are still in place.

“I think we’d better get our act together,” he said.

“Even if this one is not going to do it, there will be another avian influenza virus at the basis of the next pandemic. It is really a question of when it is going to happen, not if it is going to happen.”