An inactivated whole-virus vaccine successfully protected ferrets against the deadly H5N1 bird flu, a U.S. research team reported on Wednesday.

Unlike earlier candidate vaccines that rooted in part sequences of the virus' hemagglutinin (HA) gene, this new virus is a first recombinant virus based on both the HA and the neuraminidase (NA) genes, according to the researchers at the St. Jude Children's Research Hospital.

These findings were published in the June 15 edition of the Journal of Infectious Diseases.

The researchers led by Robert Webster created the vaccine by recombining the HA and NA genes of a virus strain acquired in Hong Kong, China, (A/HK/213/03) and weakening its virulence.

The researchers then tested the vaccine's effect using young adult ferret. One group of ferrets that received a dose of vaccine was challenged with wild-type H5N1 virus strains four weeks after vaccination. Another group that received two doses was challenged one week after the last dose.

All vaccinated ferrets were protected against lethal challenge with a most pathogenic "Vietnam" strain (A/Vietnam/1203/04), the researchers found.

"No clinical signs of infection were observed, virus replication was significantly reduced and was restricted to the upper respiratory tract, and spread of virus to the brain was prevented," they said.

Furthermore, the two-dose vaccination induced higher levels of antibodies that were cross-reactive to distinctly different H5N1 virus strains, they added.

This vaccine based on reverse genetics may meet the urgent need in a future pandemic flu that is widely accepted by scientists as inevitable, the researchers noted.

If a pandemic flu breaks out, at least six months will probably be needed to produce the first dose vaccine after the virus strain is identified. Influenza vaccines based on wild-type H5N1 virus cannot be produced on a large scale because of high virus virulence and the requirements for biosafety level, while vaccines based on less virulent strains are poorly effective.

Early stockpiling vaccines that provide cross-strain immunity may offer promise, though, the researchers said.

"The results of our study show that protection can be gained even when the vaccine strain does not match the challenge virus and that the optimal strategy for vaccination of immunologically naive populations will be the two-dose regimen, which induced more cross-reactive antibodies within the same HA subtype."

Since 2003, the H5N1 bird flu has killed millions of domestic fowls and wild birds, as well as more than 100 people in Asia, Europe and Africa. The virus can cause severe disease in humans, with multiple-organ failure and death of the infected persons.

The ongoing spread of H5N1 in the world's domestic poultry and associated transmission to humans is thought by many to be a prelude to a pandemic, in which it provides more opportunities for the virus to gain the ability for human-to-human transmission.

Source: Xinhua