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Date posted: May 22, 2012

As the number of cases of the paralytic disease fall, world health officials have to grapple with a vexing problem: a component of the most widely used polio vaccine now causes more disease than the virus it is supposed to fight

The shadows lengthen in a guesthouse cafeteria on the sprawling campus of christian Medical College, Vellore, in India. Wrapped up as he is in an issue that has possessed him for years, T. Jacob John notices neither the dying light nor the gathering mosquitoes. He is talking about the oral polio vaccine.

A slight man who speaks and moves with a speed that belies his 76 years, John is one of India’s leading polio experts. Trained as a pediatrician, virologist and microbiologist, he is also a longtime critic of the continued reliance on the oral polio vaccine—OPV in polio speak—used by the nearly 25-year-old international campaign to rid the planet of the paralyzing and sometimes fatal disease. The vaccine is at once an excellent and an imperfect tool. Inexpensive and easy to administer (each dose consists of a few drops of serum on the tongue), it has brought the world to the point where polio eradication is visible on the horizon. Indeed, the World Health Organization announced this past January that there have been no cases of naturally occurring polio in India for a year. But if the distribution of the vaccine is not choreographed with exquisite care, its continued use—at least as it is currently formulated—could actually keep the world from eliminating polio.

Today John is talking with a reporter about a problem raised by a specific component of the oral vaccine, which uses weakened viruses to elicit immunity against the three strains of polio—known as types 1, 2 and 3. (An expensive, alternative vaccine, popular in wealthy nations, consists of an injected formulation that is made up of completely inactivated, or “killed,” viruses; it is known as IPV.) The issue: type 2 poliovirus no longer exists in nature; the last case stemming from naturally circulating virus was reported 13 years ago.

Ongoing vaccination against type 2 would not be worrisome if the viruses in the oral vaccine were perfectly benign. In rare cases, however, the weakened viruses from the vaccine can revert to disease-causing pathogens and provoke the very illness they are meant to prevent. In places where wild polioviruses are still a threat, the risk from natural infection is greater than the small hazard the vaccine poses. But if the only risk of paralysis from type 2 polio comes from the strain in the vaccine itself, then that strain’s continued usage could well be considered unproductive at best and quite possibly unethical. As long as the oral vaccine contains the type 2 virus, however, children in more than 100 countries around the globe must—paradoxically—be vaccinated against type 2 polio to protect them from the type 2 virus in the vaccine.

In 2004 John wrote a letter to the medical journal the Lancet, urging the international community to remove the type 2 component from the oral vaccine, thus making it a “bivalent” vaccine that would protect against types 1 and 3 polioviruses. Like other complaints John has made about the oral polio vaccine, however, the suggestion went nowhere—until now.

The Global Polio Eradication Initiative—a partnership of the WHO, UNICEF, Rotary International and the Centers for Disease Control and Prevention—is marshalling support for an initiative to drop the type 2 component from the oral vaccine. The proposal is part of a substantial overhaul of the plan to eventually phase out the oral polio vaccine altogether once all types of wild polioviruses are demonstrated to have been extinguished. The WHO’s governing council, the World Health Assembly, will be asked to approve the early withdrawal of the oral vaccine’s type 2 component at its annual meeting in May.

If the policy change passes—and the assembly is expected to vote in its favor—the move would eliminate an ethical problem that has been bedeviling the eradication effort for years. It could also speed the job of wiping out the remaining two strains of polio in the three countries where they remain endemic (Afghanistan, Pakistan and Nigeria); a 2010 Lancet study showed that the two-target vaccine is at least 30 percent more effective than the one that has to protect against three strains of polio. And yet the poliovirus has a nasty habit of eluding efforts to contain it. Last year, for example, China reported its first cases—genetic tests traced their origin to Pakistan—in more than a decade. Adjusting the oral polio vaccine, some fear, could have unintended consequences and thus disrupt an eradication campaign that is already 12 years past its original deadline and counting.

Read more : http://www.scientificamerican.com/article.cfm?id=polios-last-act&WT.mc_id=SA_CAT_HLTH_20120515

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