One of the three serotypes of poliovirus, type 2, was eradicated in 1999, leaving health workers to contend only with types 1 and 3. However, the type 2 poliovirus has recently reemerged from a weakened form of the pathogen used in the oral polio vaccine (OPV) in Nigeria. The mutated virus’s symptoms and mode of transmission are indistinguishable from those of the wild type 2 virus, and like type 2 polio, it can only be stopped with immunization. The virus has spawned an epidemic in Nigeria, where population immunity to the “eradicated” type 2 virus is low, with 124 cases of paralysis by July 26th. Fortunately, trivalent OPV (tOPV) is highly effective against the polio type 2 virus and should curb the epidemic easily if enough children can be immunized.
The Nigerian outbreak highlights just how difficult the final stages of eradication can be, as well as the dangerous mutations of vaccine-derived virus that can occur when using live virus in vaccine. Since the smallpox vaccine is rarely administered and is not made from smallpox virus, the smallpox virus cannot reemerge; however, the Nigerian epidemic underscores the importance of taking care when working with other live-virus vaccines in order to ensure that more vaccine-derived viruses are not created.
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