Scientists have created new strains of polio intended to protect workers in factories that make polio vaccine. The new strains have the same ability to invoke an immune reaction as the live viruses now used to make vaccine do, but there is virtually no risk anyone will get polio if one of the new strains somehow escapes.
The research team, at the State University of New York at Stonybrook, is led by Eckard A. F. Wimmer, a molecular geneticist who made headlines in 1991 when he synthesized polio virus in the lab from its chemical components, the first time a virus had been made outside of living cells.
The world is very close to eliminating polio, which is now endemic to only three countries: Afghanistan, Nigeria and Pakistan. But to be sure the disease is gone, children will have to be vaccinated for several years after the last detected case.
Currently, factories making the injectable Salk vaccine used in the United States and Europe start with the dangerous wild-type viruses known as Types 1, 2 and 3. After growing a large batch, vaccine makers "kill" the virus with formaldehyde and prepare it for syringes. The finished product is safe, but if the growing live viruses ever escaped "because of a leak, an explosion, an earthquake, a tsunami, a flood," Dr. Wimmer said, "the spill could spread like wildfire."
Right now, polio eradication depends on large sweeps by volunteers putting drops of the oral Sabin vaccine into children's mouths. It is easy to give, and it produces better immunity because it reaches the intestines, which are lined with receptors for the virus.
The Sabin vaccine has drawbacks, however: it contains a still-live virus that was mutated long ago so that it is usually too weak to produce disease. In rare cases, it can mutate back into a dangerous form that paralyzes or kills. And the vaccine is risky in children with immune-system problems. For those reasons, the World Health Organization plans to eventually phase it out.
Once that happens, factories around the world will have to make millions more doses of the injectable version, so five years ago, the W.H.O. began looking for safer seed strains of virus.
Dr. Wimmer and colleagues took a part of the virus's RNA that is crucial for growth, mutated it to weaken it, and inserted it in another stretch of RNA that controls how virulent the virus is. That renders the virus less lethal. "If it were to get into the brain, it doesn't do any harm," he said.
And, he explained, even if the virus evolved to defeat that virulence-lowering mutation, it would simultaneously cripple its own ability to reproduce.
Now Dr. Wimmer's team is working with the Crucell vaccine company to prove that the safer strains grow well in Crucell's proprietary human cell line. Ideally, he said, the new vaccine will eventually be mixed with others like those for measles and diphtheria, and all will be delivered together in one painless shot by a jet injector.
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