A recent outbreak of the paralyzing viral infection polio in Haiti and the Dominican Republic has been traced to a strain of oral polio vaccine (OPV) that mutated back to virulence, according to international health officials.
Based on genetic analysis of viral samples, they believe the outbreak, which struck nearly two dozen children in both countries between 2000 and 2001, arose from OPV given to one child in 1998-1999.
Poliovirus spreads from person to person, usually going unrecognized because it produces mild symptoms or none at all. When it attacks nerve cells, however, the infection can cause crippling, sometimes deadly, disease.
OPV contains a live, weakened polio virus that is very effective at conferring immunity to the infection. But it can, in rare cases, cause polio. For this reason, countries such as the US now use only inactivated polio vaccine (IPV), which is injected.
OPV continues to be the standard in developing countries, however, because it is cheaper and easier to administer, doesn’t require supplies of sterile needles and is more effective than IPV at preventing outbreaks.
But the cases in Haiti and the Dominican Republic illustrate a potential risk with OPV when it is given in a population where many people are unvaccinated. After a person receives OPV, virus from the vaccine is shed in the stools for a short period of time. In this outbreak, shed virus from a single OPV dose spread and mutated back to a virulent state, causing paralytic disease in a group of children who had either not been vaccinated or had not received a complete course of OPV.
This same scenario has been blamed in a number of cases of paralytic disease in Egypt and the Philippines. According to the researchers, one of the critical factors in all of these outbreaks was the large number of unvaccinated, vulnerable people in the population.
In fact, it is common for OPV strains to “back-mutate.” It only becomes a potential problem in populations with low vaccination rates, which lack a “wall of immunity.”