Washington - U.S. government researchers said on Thursday they had identified the structure of the "bird flu" virus that has killed six people in Hong Kong and said their finding could help efforts to formulate a vaccine.
Although there have been no new cases of infection by the H5N1 virus in the past two weeks, they said it was important to find out why it infected more than a dozen people.
Kanta Subbarao of the U.S. Centers for Disease Control and Prevention (CDC), along with colleagues there and at the U.S. Department of Agriculture, examined samples of the virus taken from the first known victim, a three-year-old boy who died in Hong Kong last year.
Subbarao, head of the CDC's molecular genetics lab, said the good news was that the virus is clearly a bird virus, and there were no signs it had swapped genes with human flu viruses -- the biggest danger.
Flu viruses are so hard to fight because they can exchange genes with one another, creating new strains that the body is hard put to fight against. This is why a new strain of flu sweeps the world almost every year, some more dangerous than others.
Scientists believe most start out in birds, but bird flu is very different from human flu and normally cannot infect people directly. Pigs can be infected by both avian- and human-type flu viruses, and often act as an intermediate mixing bowl for influenza genes.
China has many pigs and lots of poultry, as well as a population in close contact with both, which is why many strains of flu seem to originate there.
But H5N1 is clearly a bird virus, Subbarao said.
And there was no evidence the virus had yet shuffled its genes with a human strain. Further study would show how it ever infected any people in the first place.
The key to the structure lies in two proteins that flu viruses carry, known as hemagglutinin (HA) and neuraminidase (NA) proteins. Differences in these two proteins characterize different strains of influenza.
"Lack of protective immunity in the human population against the new HA and NA proteins can result in rapid global spread of the virus, leading to widespread morbidity (illness) and mortality," Subbarao's group wrote in their report, published in the journal Science.
They used polymerase chain reaction (PCR) to create many copies of the HA and NA genes so they could look at them.
Analysis showed a structure of amino acids that is associated with highly pathogenic viruses -- which are not only very infectious but which are deadly.
Hong Kong health authorities said earlier Thursday the "high risk" period had passed, with no new cases reported since a mass slaughter of poultry ended two weeks ago.
A team of international health experts were headed for China to check the population to see if anyone else has been infected, and to check out chicken farms.
CDC officials say their first priority would not be to vaccinate people against the new flu, but rather to vaccinate chickens. Most flu vaccines are made using egg whites from chicken eggs but because H5N1 is so deadly to the birds, such standard formulations don't work.
Even if it does not threaten the human population, the virus could wipe out poultry worldwide, the CDC's Charles Beard said.
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