Scientists at the U.S. Centers for Disease Control and Prevention (CDC) have found that just two small changes in the virus that caused the 1918 Spanish flu pandemic stopped it from jumping between mammals in the laboratory.
Dr. Terrence Tumpey, who led the research says they believe their finding could help determine what will cause the next pandemic.
The researchers who have been studying a reconstructed version of the 1918 virus found it very easy to stop it from spreading from one infected ferret to another, although the altered viruses still quickly killed the animals.
Although the finding does not reveal whether or how the dreaded H5N1 bird flu virus would change to be able to pass between humans, it does illustrate that a certain type of biological change may be crucial in causing pandemics.
CDC Director Dr. Julie Gerberding says such research is vital as the more that is learned about what may have contributed to the spread and deadliness of the 1918 pandemic will mean better preparations can be made for the next pandemic.
World experts have been concerned for some time that a pandemic could happen at any time and the most likely suspect is H5N1 avian influenza currently circulating.
The H5N1 strain of the virus has to date killed over 160 people worldwide, and resulted in the deaths either by infection or by preventative culling, of millions of birds and poultry.
Influenza presents such a problem because the virus mutates regularly, is easily spread and kills people.
According to the World Health Organization seasonal flu claims the lives of between 250,000 and 500,000 people a year and every 30 years or so, along comes a strain which will make a big mutation and suddenly become more deadly.
The 1918 pandemic, caused by the H1N1 virus, was responsible for 50 million deaths worldwide over a two year period.
The current strain of H5N1 has killed more than half of its human victims.
The only positive at present is that its current form does not infect people easily and as been almost always been transmitted via infected birds or poultry.
If the scientists can establish which mutations would render H5N1 able to pass easily from person to person they might then be able to predict or even control the spread of the virus.
When the CDC team reactivated the 1918 H1N1 virus they infected caged ferrets with either the original flu virus (reconstructed two years ago) or the altered form.
The cages of the infected ferrets were later placed next to healthy animals to see if the virus would spread through the air.
Tumpey says they found that the 1918 virus spread very efficiently while the altered virus did not spread at all.
Tumpey, a senior microbiologist at the CDC, says by using genetic engineering they made a modest change of two amino acids in the hemagglutinin protein (the "H" in an influenza virus name) that stopped the transmission of the virus between the ferrets.
Therefore it is possible that the hemagglutinin, found on the surface of flu viruses, is what enables it to transmit efficiently from one person to another.
In order for an influenza virus to spread efficiently, it needs to attach itself to the cells in the human upper airway instead of to cells in the gastrointestinal tracts of birds and somehow the H1N1 bird virus adapted itself to humans.
H5N1 is a bird virus that has not as yet made this change and Tumpey suspects changes in more than one virus protein would be needed for the H5N1 virus to be transmitted among humans.
The researchers say the mutations that enabled H1N1 to move from birds to people do not make H5N1 shift in the same way.
The research is published online in the February 1 issue of Science.