H5N1 vaccine strain in a week

Using reverse genetics, WHO thinks a prototype bird flu strain likely to be ready in a week

Jan 29, 2004
Robert Walgate(walgate@scienceanalysed.com)

GENEVA—A prototype vaccine strain of the H5N1 flu virus causing havoc in Asia will probably be ready next week, John Wood of the UK National Institute for Biological Standards and Control (NIBSC) told The Scientist today (January 29). However, months of other hurdles remain before it may be ready for public health use.

As leaders of the countries worst hit by the avian flu met in Bangkok at an emergency summit on Wednesday (January 28), World Health Organization (WHO) labs were working to formulate a vaccine against the virus.

The H5N1 virus kills chicken eggs, the normal medium for growing flu vaccine viruses, so the WHO laboratories are using reverse genetics to lower the pathogenicity of the virus to chickens and to get a high yield in the egg cultures, said Klaus Stöhr, project leader of the influenza surveillance and scientific groups for the H5N1 outbreak team. Reverse genetics also cuts down the normal time required for flu vaccine production.

Three WHO collaborating centers—The US Centers for Disease Control and Prevention, St Jude's Children's Research Hospital in Memphis, and the UK's NIBSC—are now using the technology to create a prototype H5N1 vaccine virus. The focus of attention in vaccine strain production is the hemagglutinin gene, said Stöhr.

Wood says work with the virus sample from Vietnam is being done under strict P4 conditions, “or it would kill the staff.” His team will remove a stretch of 4 or 5 basic amino acids at the hemagglutinin cleavage site that allows the virus to replicate in every organ of a chicken's body, rather than respiratory and gut tissue normally infected. "We are making a jump to guess this is also important in humans," he said.

The NIBSC team extracted the viral RNA last week, said Wood, and is now growing the appropriate plasmids in the laboratory. Using other lab strain flu plasmids containing the other components of the viral genome, the team will then reassort the pieces into a nonpathogenic vaccine strain. “We hope to 'rescue' the reassortment virus next week,” said Wood.

After that comes amplification in embryonated hen's eggs, followed by safety testing in chickens and in ferrets.

Sufficient amounts of safety-tested prototype vaccine virus will probably be available for the necessary 1 to 2 months of clinical trials in the next 4 weeks, Stöhr told The Scientist.

Large-scale commercial production requires negotiation with MedImmune, the US company that holds the patent for the technique, he added, but the company indicated to the WHO in April last year that it would be willing to do this “and that they would charge very moderate licensing fees, and consider special conditions for developing countries. They';ve told us they would not want any country to be unable to produce an affordable vaccine.”

WHO would probably help coordinate the trials to assess what the antigen concentration will be, how many doses should be given, what adjuvants may help, or how each human age group will react. “And they could be done in parallel with the production of clinical batches, so production could begin immediately,” he said.

The next hurdle would be regulatory approval. “As the European Union may consider this to be a genetically modified organism, that would impose much higher safety standards during manufacture, until inactivation,” Stöhr believes. “But the potential danger of not having a vaccine ready might influence their decision. They will have to look at the losses and benefits.”

The reason such urgency is being given to the vaccine production is the fact that the eight genes of the H5N1 avian influenza could shuffle with the eight genes of human flu in a coinfected individual, with the potential to create a human pandemic.

“But we don';t want to create panic,” Stöhr told The Scientist. “We have a window of opportunity to keep this in the box,” by slaughtering the infected animals, he said.

In cells infected with influenza, the viral genome dissociates and the separate genes multiply and recombine to make new virus particles. If both human and avian viruses are present in a cell, all possible recombinations can be made, potentially uniting the pathogenicity of the current H5N1 virus with the infectivity and transmissibility of normal human flu.

WHO is unwilling to estimate the global mortality if the H5N1 virus recombines to make a human form. In 1918, there were 20 to 40 million deaths in the infamous flu pandemic, but it is argued that many of those were caused by secondary bacterial infections, which could nowadays be more easily handled with antibiotics.

On the other hand, says Mike Ryan, director of WHO's Global Outbreak Alert and Response Network, the argument is contentious. “One of the striking facts about that pandemic is the number of perfectly healthy individuals who died within 2 to 3 days of getting sick,” he said.

According to the WHO';s most recent update, only two countries, Vietnam and Thailand, have reported laboratory-confirmed cases of H5N1 infection in humans. Vietnam has reported eight cases, six of them fatal. Thailand has reported three cases, two of them fatal.