Vaccine-derived polioviruses.

The Sabin oral poliovaccine (OPV) is extremely efficacious and safe, despite its inherent genetic instability. While reversion to nearly wild-type phenotype regularly occurs soon after the onset of OPV reproduction in the gastro-intestinal tract of vaccine recipients or their contacts, this is usually not a big problem, provided the vaccine is used either for mass vaccination or in populations with a relatively high level of anti-polio immunity. However, if these conditions are not met, the vaccine viruses are likely to be converted into highly transmissible agents with a nearly wild-type level of neurovirulence. Moreover, OPV viruses may persist and evolve even in adequately immunized populations. The current strategy for the "endgame" of poliovirus eradication envisions cessation of OPV usage shortly after the last isolation of a wild poliovirus. If implemented, this strategy would result in rapid growth of non-immune human populations at the time when OPV derivatives would very likely be persisting. Therefore, the planned cessation of OPV vaccination is associated with a very high, and in the author's opinion, unacceptable risk of polio outbreaks caused by OPV derivatives. The only currently available tool to curb such outbreaks is OPV, which should have been used at a global scale. Safe discontinuation of OPV vaccination will be possible only after an efficient new vaccine or an anti-poliovirus drug is available. To achieve this goal, stimulation of poliovirus research and elimination of organizational and financial obstacles preventing it are needed.