Live Bird Flu Virus Vaccine Protects Animals, Study Shows
WASHINGTON Three experimental vaccines using live but weakened versions of the H5N1 bird flu virus appeared to protect animals from infection, and might offer a way to stockpile vaccines ahead of a pandemic, U.S. researchers said Monday.
They said the approach was already being tested in people, and might provide the start of a repository of vaccines against various potential strains of pandemic influenza.
"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," the researchers wrote in the online journal Public Library of Science-Medicine.
Unlike some of the other experimental vaccines being developed, it takes only a single dose of a live, weakened vaccine to stimulate a good immune response, the researchers said.
Such vaccines induce what is known as cross-protection, meaning the vaccine protects against other, similar strains of the virus, the researchers said. This would be useful because the current flu mutates a little every year, forcing vaccine makers to reformulate annually.
"If an influenza pandemic were imminent or underway, we would need a vaccine that could stimulate immunity quickly, preferably with a single dose," said National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci.
Experts fear the H5N1 avian flu virus now killing birds globally could evolve into a form that easily infects people, sparking a pandemic that could kill millions.
Although H5N1 has infected just 244 people and killed 143, governments, companies and other organizations are racing to produce a vaccine.
Some of the vaccines use pieces of DNA from the viruses and others use a virus that is completely inactivated, or killed. Most seasonal flu vaccines use a virus that has been killed, an approach that offers little cross-protection.
Maryland-based MedImmune Inc. takes a different approach, engineering a live but weakened virus that is delivered as a nasal spray instead of injected by needle.
MedImmune researchers worked with teams at the National Institutes of Health and the U.S. Department of Agriculture to make a live H5N1 vaccine. The teams, led by Dr. Kanta Subbarao of NIAID, used three different strains of H5N1 dating back to when the dangerous form first emerged in Hong Kong in 1997.
Mice that got a single dose of vaccine, given in a nose spray, all survived normally lethal doses of H5N1, the researchers reported. Mice and ferrets given two doses of vaccine were protected and their bodies also suppressed the virus, the researchers found.
The team artificially constructed their viruses using weakened flu strains and added key proteins from H5N1 strains that infected people in 2004, 2003 and 1997.
But they used the usual low-tech approach to produce the vaccine, growing it in chicken eggs -- the same way 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").
"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) ..." they wrote.