According to US researchers three experimental vaccines using live but weakened versions of the H5N1 bird flu virus seemed to offer protection against infection, and could probably be the solution to to stockpile vaccines ahead of a pandemic.
They said tests on people were already underway, and this could lead to the start of a repository of vaccines against various potential strains of pandemic influenza.
The researchers wrote in the online journal Public Library of Science-Medicine, "We have been developing live, attenuated influenza virus vaccines because they have properties that make them attractive vaccines for the prevention of pandemic influenza in humans."
Another advantage that lay with such vaccines is the benefit of cross-protection, which meant that the vaccine protects against other, similar strains of the virus. This was useful especially because the current flu mutates a little every year, forcing vaccine makers to reformulate annually.
Director of National Institute of Allergy and Infectious Diseases, Dr. Anthony Fauci said, "If an influenza pandemic were imminent or underway, we would need a vaccine that could stimulate immunity quickly, preferably with a single dose."
Health experts fear a mutation of the H5N1 avian flu virus that is now killing birds globally, into a form that easily infects people, sparking a pandemic that could kill millions.
Although H5N1 has infected only 244 people and killed 143, governments, companies and other organizations are racing to produce a vaccine.
So far the vaccines that have been developed use pieces of DNA from the viruses while some others use a virus that is completely inactivated, or killed. Since most seasonal flu vaccines use a killed virus there is little chances of cross-protection.
Maryland-based MedImmune Inc has engineered a live but weakened virus that is delivered as a nasal spray instead of injected by needle.
Researchers from MedImmune worked with teams at the National Institutes of Health and the U.S. Department of Agriculture to make a live H5N1 vaccine. The teams were led by Dr. Kanta Subbarao of NIAID using three different strains of H5N1 dating back to when the dangerous form first emerged in Hong Kong in 1997.
The researchers reported that mice that got a single dose of vaccine, given in a nose spray, all survived normally lethal doses of H5N1. It was found that mice and ferrets given two doses of vaccine were not only protected but that their bodies also suppressed the virus.
The team artificially constructed their viruses using weakened flu strains and added key proteins from H5N1 strains that infected people in 1997,2003 and 2004.
Even more amazing was the usual low-tech approach to produce this vaccine, growing it in chicken eggs, that is in the same way that seasonal flu vaccine is produced.
MedImmune said in June it was already testing one of the vaccines in human volunteers, in what is known as a Phase I safety trial.
The researchers noted it is not possible to predict which strain of H5N1 or any other influenza virus might cause a pandemic. There are hundreds of different possible combinations of hemagglutinin (the "H" in a flu strain's name) and neuraminidase (the "N").
The researchers wrote, "If the vaccine candidates described in this paper elicit a broadly cross-reactive protective immune response in humans, they would support the approach of developing one or two pandemic vaccine candidates for each subtype (H4 through H16) ...."