A vaccine designed to protect children from polio might have instigated a recent flare-up of the disease in the Dominican Republic and Haiti--the first outbreak of polio in the Western Hemisphere in more than 9 years. This is the first reliable report that a vaccine-strength polio strain may have reverted to a virulent form and spread contagiously. A massive vaccination campaign already in the works is expected to contain the outbreak.
The oral polio vaccine is highly effective and easily administered. But because it employs live but weakened strains of the virus, it causes vaccine-associated paralytic polio in about 1 of every 750,000 people who receive it, usually those with compromised immune systems. There has been no evidence that this vaccine-induced form of the disease is contagious.
Now an outbreak on the island of Hispaniola demonstrates that the vaccine can mutate to a virulent form and spread. Three children in a rural area of the Dominican Republic and another child in a remote area of Haiti came down with paralytic polio in July and August. None of the children had been vaccinated against the disease. Laboratory analysis, first by a local lab and then by the U.S. Centers for Disease Control and Prevention in Atlanta, showed that the children had been infected by a vaccine-derived virus that had become virulent. The Pan American Health Organization (PAHO), which announced the findings 1 December, has convened a group of scientists to study the data and recommend action.
The World Health Organization, of which PAHO is one division, is striving to eradicate polio from the world by 2005. Whether this incident will delay the timeline remains to be seen, says Donald Henderson, an epidemiologist at Johns Hopkins University in Baltimore, Maryland, who led WHO's successful global eradication of smallpox. "We really want to zero in on this and check it out in great detail," Henderson says. "This comes as a great surprise to everybody."