It is too early to stop polio vaccination.

The paper by Wood, Sutter and Dowdle presents a balanced discussion of the hurdles facing the eradication effort (1). However, there are significant omissions, and many questions remain unanswered. The WHO Global Plan of Action for Laboratory Containment of Wild Polioviruses (2) calls for cessation of vaccination at some point after eradication and containment. An important question is whether vaccine virus excreted from vaccinees will persist and constitute a threat to unimmunized infants. There are some data from a few countries on the extent of poliovirus vaccine persistence in the population. Some have concluded from these data that persistence is of limited duration, but in this writer's opinion the data are far too limited to justify this conclusion. For example, there is a report of `silent' circulation of poliovirus type 2 in Israel (3), and this phenomenon might be more widespread. An even greater problem is prolonged shedding of poliovirus by immunocompromised vaccinees. It is already known that they may harbour poliovirus for up to 10 years. It seems unlikely that all such persons can be identified. Certainly, no provision for doing so is included in the Global Plan of Action. It can only be concluded that immunocompromised vaccinees will be a continuing threat in the post-immunization era. Wood et al. describe a number of ongoing studies designed to characterize the transmission of poliovirus vaccine strains (1). It is hard to imagine how such data could ever rule out the possibility that a shed vaccine virus might constitute a threat to unimmunized individuals. For example, mutations might arise during circulation of poliovirus that enhance endemic transmission. Unfortunately, there are no animal models with which to study poliovirus transmission. Transgenic mice that express the poliovirus receptor are not susceptible to infection by the oral route. It might be feasible to engineer transgenic mice that are orally susceptible but such a result is likely to be several years away, and it is not guaranteed that such animals would be suitable for studying the molecular basis of poliovirus transmission. An important component of the eradication effort is the identification and destruction of poliovirus stocks, to prevent reintroduction of the virus in the post-vaccination era. Not enough attention has been paid to the lessons and revelations that have come out in the past few years about smallpox eradication. In that case, a virus that had been very tightly controlled in a small number of laboratories is now perceived as the number one bioterrorism threat in the word (4). Wood et al. do not indicate why poliovirus would be easier to contain or restrict than smallpox virus. There is no discussion of the mind-boggling scale of tracking down all poliovirus stocks, particularly in light of the absence of an enforcement authority. Wood et al. state that laboratories with known poliovirus stocks can be `relatively simply identified through surveys'. This opinion is disconcerting: one could easily imagine, for example, that certain countries would prefer to keep their poliovirus stocks for biological warfare. The problem of identifying laboratories that harbour poliovirus without knowing it is an even greater challenge, and seems unsolvable. And how do we deal with the situation in which a tube labelled `Coxsackievirus B3' actually contains poliovirus type 2? Since such a situation has actually occurred (5), it is not a hypothetical threat, but a real possibility. The suggestion by Wood et al. that `goodwill' and the `thoroughness with which ... nations exercise their responsibilities' will make the effort succeed is simplistic. A more detailed plan should be produced. One possible reservoir for the poliovirus genome is through recombination with other enteroviruses. Wood et al. conclude that `recombinants are unlikely to pose a threat to the eradication programme'. One basis for this statement is that `only recombinants carrying the wild-type poliovirus capsid' would be virulent. …