Labs rush to cultivate bird flu vaccine Reverse genetics allows creation of weakened virus

Sabin Russell, Chronicle Medical Writer
Thursday, January 29, 2004

If a lethal influenza now striking birds in Asia should change into a strain that attacks humans, the health of millions may depend on an untried genetic engineering technology sprung from U.S. academic laboratories and owned by a company with Bay Area roots.

The World Health Organization in Geneva has launched a crash program to develop a vaccine against the bird flu using a process known as reverse genetics that allows researchers to construct in the lab a radically weakened version of the lethal virus, suitable for mass-production in vaccine factories.

As of Wednesday, the bird flu had hit poultry farms in at least 10 Asian countries and killed 10 people. Although there is no evidence that the bird flu can spread between human beings, scientists worry that, with a few quick evolutionary turns, this virus could mix with human flu strains and trigger a global pandemic.

"The ingredients for an influenza pandemic exist in Asia,'' said Dr. Klaus Stohr, director of the World Health Organization's influenza response program.

The problem facing public health officials is that the traditional method for developing a vaccine -- which involves culturing the virus in fertilized hen eggs -- has so far proved difficult with the new strain in Asia, known as H5N1.

Reverse genetics may be essential for a quick response to the bird flu. The new technique could essentially redesign the bird strain so that it not only grows well in eggs but poses no threat to employees in vaccine factories. If successful, it could supplant technologies used since the 1940s to develop flu shots.

Scientists at the federal Centers for Disease Control and Prevention in Atlanta are using reverse genetics to produce a vaccine strain of the Asian bird flu. Similar efforts, coordinated through the U.N. health agency, are under way at labs in London and in Memphis.

Mass production is still likely to occur using the traditional method of culturing the virus in eggs, killing the virus and purifying pieces of the virus' outer coat, which stimulates protective antibodies when injected into the arm. But even with the new technology, large-scale production of a protective vaccine is still months away.

According to Stohr, it will be two to three months before the laboratories produce a vaccine prototype, and further tests will be needed to assure its safety. "It will still take four to six months before a significant amount of vaccine can be produced,'' Stohr said during a telephone briefing with reporters Wednesday from Geneva.

Experts note reassuringly that the Asian bird flu is currently almost exclusively confined to chickens and ducks. Humans living in proximity to birds have gotten sick, but the numbers are low, and they are not transmitting the disease to others.

What worries health experts is that the disease is spreading among birds throughout Asia. The more birds that have it, the greater the chance a version that spreads among humans will emerge.

"The real issue now is to avoid the emergence of a pandemic, and that means to get rid of -- to eliminate -- the animal reservoir," Stohr said.

Millions of chickens are being slaughtered in Vietnam, Thailand, Japan, South Korea and China in a bid to snuff out the bird virus.

The consequences of a bird flu's taking hold in the human population are horrific. Because the strain is so different from the influenza viruses that have swept through human populations in the past, there is thought to be very little natural immunity to it. This flu has also proved to be extremely deadly to the few people who contracted it directly from birds.

As a precaution, therefore, efforts also are under way to prepare a vaccine should this lethal bird flu begin to spread within the human population. Eleven international pharmaceutical companies are considering manufacture of the flu vaccine.

The creation of a genetically engineered flu vaccine relies on techniques first developed in university laboratories such as that of Peter Palese, chairman of Microbiology at the Mount Sinai School of Medicine in New York. Major advances were later accomplished by Dr. Yoshihiro Kawaoka of the University of Wisconsin and by Dr. Robert Webster of St. Jude Children's Hospital in Memphis.

Much of the know-how in reverse genetics for flu vaccine is covered under patents held by MedImmune Inc., a Gaithersburg, Md., biotechnology company. It acquired those rights when it purchased Mountain View vaccine maker Aviron in 2002 in a stock deal valued at $1.5 billion. Aviron had purchased the rights from Mount Sinai in 1993.

Under an agreement struck with the World Health Organization last year, MedImmune has agreed to license the reverse genetics technology to other drugmakers to test new vaccines in the event of a pandemic. MedImmune spokeswoman Jamie Lacey said Wednesday that the company had offered licenses "for very reasonable terms" to vaccine makers "as part of our commitment to public health.''

MedImmune is better known for its development of a nasal flu vaccine that is made with an entirely different technology. Despite high expectations, sales of the company's FluMist vaccine have been disappointing. The vaccine costs roughly 10 times that of a conventional flu shot.

Reverse genetics allows researchers to assemble designer flu viruses, using biotechnology tools to remove viral traits that make a particular strain lethal, while retaining components that stir antibody responses from the human immune system.

Flu viruses use as their genetic code a chain of amino acids known as RNA, which is more fragile than DNA, which carries the genetic blueprints of higher life forms. Reverse genetics converts the eight influenza RNA genes into DNA, which can be readily manipulated in the lab to add or remove traits.

"What is being made is identical to what comes out of the classic genetics process, but with more control over the safety, accuracy and speed of the process,'' said Dr. Harry Greenberg, associate dean for research at Stanford University School of Medicine and a consultant to MedImmune.

The goal of World Health Organization flu vaccine designers is to take the outer protein coat of the bird flu virus and cripple the subtle surface traits that appear to make it particularly deadly. The altered outer coat is grafted onto the core of a 1934 flu strain that, for decades, has reliably grown in eggs. One risk is that there is no certainty that the vaccine developed from the current bird flu strain will work on a strain that mutates into a human pathogen. In order for the bird flu to become a human pandemic, the virus will have to change some of its current traits. A change of only two amino acids in the flu bug that struck California in November may have rendered the existing flu vaccine useless against it.

But Linda Lambert, acting section chief for SARS and flu at the National Institute of Allergy and Infectious Diseases, said chances were good that to the human immune system, a pandemic strain would still look very similar to the genetically engineered vaccine strain.

Vaccines stir up antibodies that attack the outer coat of the flu virus. The genetic changes that might cause the bird bug to become dangerous to humans could involve the inner core. So a vaccine against the bird flu might well match an emerging human pandemic strain. "People are starting out with the hope that we will be lucky,'' she said.