Risks of reintroduction of polio after eradication: the vaccine origin of an outbreak of type 3 poliomyelitis.

Sabin live-attenuated strains, which have proved to be the most effective tools for poliovirus eradication, could also be the source of reintroduction of polio epidemics after global eradication of wild poliomyelitis is achieved. There are still considerable gaps in our knowledge about the persistence of vaccine-derived viruses in the population and the mechanisms involved in poliovirus transmissibility, both of which are essential factors in assessing the risks posed by such strains and in designing effective strategies for the cessation of polio immunisation. In this report, we have examined virological and epidemiological aspects of an epidemic of poliomyelitis in 1968 in Poland that was shown to be associated with the use of the USOL-D-bac live-attenuated vaccine strain. Possible causes of the origin and progress of the outbreak included the pattern of virus excretion from vaccinees, mutations identified in epidemic viruses and the unique vaccination policies in Poland during the years preceding the epidemic.