Gaps in our knowledge about transmission of vaccine-derived polioviruses.

It has long been known that oral poliovirus vaccine (OPV) viruses are transmissible ¿to some degree¿ between vaccinated individuals and their contacts. This property has generally been considered an advantage of these vaccines, since it can spread immunity, despite the fact that the viruses sometimes revert to virulence and hence can also cause vaccine-attributable paralytic poliomyelitis in vaccines and their contacts. However, several gaps in the knowledge about transmission of vaccine-derived polioviruses are noted. These include questions regarding the potential persistence of OPV viruses, transmissibility measure, correlation between humoral and intestinal immunity, rates of selection, prevalence of the viruses in stored clinical samples, and sensitivity of environmental sampling for OPV viruses. In view of this, intensive research should be conducted to gather as much relevant information about the transmissibility of the viruses under various conditions. Also, cessation of vaccination should be organized in such a way to minimize the probability of continued transmission. Moreover, stringent measures should be taken up to prevent the future reintroduction of the viruses.