| Abstract: | A small cluster of polio cases in the Dominican Republic and Haiti marks both the first polio outbreak in the Western Hemisphere in more than 9 years and the first reliable report that a vaccine-derived polio strain may have reverted to a virulent form and spread contagiously. Although a massive vaccination campaign already in the works is expected to contain the outbreak, the unusual incident raises troubling questions about a vaccine that has been in widespread use for nearly 40 years. |
