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Polio Vaccination Strategies Assessed as Eradication Nears

December 20, 2005

Polio is on track to become only the second disease ever eradicated. In two studies in the Dec. 15 issue of The Journal of Infectious Diseases, now available online, scientists are working to ensure that once it is gone, it stays gone. One study reduces concerns that people whose immune systems were weakened by HIV would re-introduce poliovirus into the community. The other study looks at the how switching forms of vaccine from a live, attenuated vaccine to an inactivated version may affect communities.

Oral polio vaccine (OPV), one of the vaccines instrumental in driving the disease to near-eradication, contains weakened live virus strains. The vaccine is highly effective, easy to administer, relatively inexpensive, and has been used for more than 40 years. Those given the vaccine excrete, or "shed," virus in their stool. There is some concern over the use of OPV, however, because vaccine-derived poliovirus (VDPV) can occasionally cause another form of polio.

Furthermore, in rare cases, immunodeficient persons have shown prolonged shedding of VDPV, which may be transmitted to contacts, thus potentially re-introducing polio into the community. Concerns had been raised that this issue would be a particular challenge in countries with high HIV prevalence.

Karen Hennessey, PhD, MSPH, and colleagues in Cote d'Ivoire and at the Centers for Disease Control and Prevention examined the duration of shedding of oral poliomyelitis vaccine by individuals with HIV infection.

Dr. Hennessey and colleagues tested stool specimens at various intervals following vaccination. Out of a total of 419 adults with HIV infection, no poliovirus was isolated from any of the specimens. Because of these results, it is likely that fewer than 1 percent of adults with HIV infection experience prolonged virus shedding when exposed to OPV, "and therefore probably represent minimal risk of re-introducing vaccine virus into the population after poliovirus has been eradicated," the authors concluded.

The alternative to OPV, a live virus vaccine, is an inactivated polio vaccine (IPV). IPV is safer because the inactivated virus in IPV cannot mutate into paralytic VDPV. However, the immune response generated in the digestive tract by IPV was believed to be less potent that that of OPV. Therefore, although those immunized with IPV would be protected from polio, they could still be infected and shed virus, contributing to the spread of the disease.

But all research on the subject had been performed using only one dose of a vaccine that was less potent that the currently used version. Konstantin Chumakov, PhD, and colleagues at the Food and Drug Administration and various other institutions throughout the United States revisited the issue using two doses of the newer, enhanced vaccine.

They found that two doses of IPV did produce immunity in the digestive tract. The researchers used shedding of virus after treatment with a dose of OPV to measure the immune response. Seventy-five percent of children receiving two doses of IPV, followed by one dose of OPV, shed OPV virus a week later, compared to 92 percent of those receiving just one dose of OPV. After three weeks, 81 percent of the children receiving one OPV dose were still shedding virus, while only 37 percent of those immunized with IPV first were. The amount of virus shed was also lower in those receiving IPV.

However, the mucosal immunity generated by IPV was less than that achieved by OPV. After two doses of OPV, only 22 percent of children given a third dose of OPV were shedding virus after a week. That figure dropped to 5 percent after three weeks.

The researchers concluded that IPV-immunized communities are partially protected from spread of poliovirus. Better protection may be achieved by enhancing IPV further, they said.

However, an accompanying editorial by Harry F. Hull, MD, and Philip D. Minor, MD, at the Minnesota Department of Health and the U.K. National Institute for Biological Standards and Control noted that "the time and expense of developing, testing and securing regulatory approval for a new IPV makes it unlikely that such a vaccine could be available on a timely basis." They also discussed public health, ethical, and financial reasons for stopping vaccination once poliovirus is effectively non-existent, and the risks associated with doing so. Whereas Hull and Minor blame politics and complacency as ultimately being the biggest threats to polio eradication, they conclude that choosing an appropriate vaccination strategy to prevent poliovirus re-introduction is an important and unavoidable task.

The Journal of Infectious Diseases