August 1997

Should Polio Vaccinations End?
Experts Still Debating

Scientists declared the Western Hemisphere "polio-free" in 1994; two years later, only 4,000 cases were reported worldwide. Wild polio virus has been largely confined to South Asia and Africa; complete eradication of the disease is expected by 2003.

As a result, World Health Organization (WHO) officials are now exploring strategies for bringing an end to polio vaccinations.

In a "Policy Forum" in the August 8 issue of Science, one pair of scientists at WHO in Geneva and another pair at Columbia University College of Physicians and Surgeons in New York square off on some of the issues involved.

This policy debate comes less than one month before a planned September 1997 meeting convened by WHO to draft "a plan for containment procedures that will be widely circulated in the scientific community before implementation."

The Columbia scientists, A.W. Dove and V.R. Racaniello, question whether it will ever be possible to end polio vaccination.

• For one thing, they say, the eradication campaign is using only the live Sabin oral vaccine (OPV), because it is cheaper than the inactivated vaccine and does not require trained personnel and sterile needles -- resources that many developing countries just don't have.
• Anyone vaccinated with live virus will shed some of that virus into the environment, they warn, and since "the Sabin strains mutate readily back to virulent forms, potentially pathogenic viruses are still being released into the aquifers."
• Furthermore, they say, since poliovirus stocks are distributed among hundreds, or even thousands of sites, "it is unlikely that all virus stocks can be found and destroyed." This means that accidental -- or even intentional -- release of the virus from laboratory stocks could occur.
• They add, "Even if total virus destruction could be accomplished, the small size of the poliovirus genome, whose sequence is known and whose complementary DNA is infectious, would make it possible for a terrorist to synthesize a new stock."

Their conclusion:

"In the post-vaccine world, the susceptible population would increase each year and the large number of potential sources of reintroduction would soon constitute a major threat." Basing their calculations on a 1992-93 Dutch epidemic, they roughly estimate the magnitude of the threat this way: "In a city of 10 million unvaccinated individuals ... a single release of virus could result in 7000 paralytic cases."

Harry F. Hull and R.B. Aylward at WHO question the view that there is too great a potential for polio persistence with the use of live vaccine.

• "Eventual cessation of control measures is inherent to the concept of disease eradication," they write, continuing, "it has always been anticipated that immunization against poliomyelitis would be stopped."
• They cite a study that calls poliovirus survival finite, "with survival in the environment lasting months rather than years."
• They add that inventory, control and containment of all clinical materials is manageable, paling in significance as a task "when compared with tasks already accomplished" in reducing polio around the world to its current low level.
• They recognize that "some immunocompromised individuals have excreted vaccine strains for 2 years or more," but state that "there are no data to show that vaccine viruses persistently circulate in a general population or cause outbreaks of paralytic polio myelitis."
• They warn that any decision to replace OPV with IPV (inactivated polio vaccine) "cannot be made lightly," adding that "Operational IPV production capacity is quite limited and would have to be expanded for IPV to be used worldwide."

Hull and Aylward conclude, "Halting the use of OPV remains the simplest and most cost-efficient means for stopping immunization. This option should not be abandoned without compelling reasons to do so."