Following certification of polio eradication by the year 2005 or shortly thereafter, the public health community and policy makers will be faced with the decision of how and when to stop polio vaccination. The benefits of ceasing vaccination are well defined (i.e., annual savings of U.S. $1.5 billion in direct global vaccination costs; the possibility of directing these savings to other health priorities; and eradication of vaccine-associated paralytic poliomyelitis cases). The risks are obvious. If poliovirus is reintroduced into a susceptible population, a catastrophic epidemic of paralytic disease, disability, and death could ensue. Poliovirus could reemerge through 1) reintroduction of poliovirus from a laboratory; 2) prolonged replication in immunodeficient patients; and 3) persistent transmission of vaccine-derived virus in populations. The probability of vaccine-derived poliovirus remaining in circulation is impossible to estimate directly. However, the basic reproductive number or BRN (derived from proportion excreting virus, duration of excretion, virus titer in stool) for vaccine-derived poliovirus is lower than for wild-type polioviruses (typically between 2 and 5 in industrialized countries and 10 and 15 under conditions of poor hygiene in tropical countries). Additional data exist from outbreak investigations, molecular sequencing of polioviruses, and studies of poliovirus persistence following mass vaccination campaigns (e.g., Hungary, Finland, and Cuba). Although the currently available data are encouraging (i.e., decreasing BRN and 3 to 6 months' duration of circulation), substantial gaps in knowledge still exist, including the probability of continued virus circulation in populations with poor hygiene. These gaps need to be addressed to ensure that the best available scientific data will be available for decision-making.

Global documentation of laboratory containment of materials infected or potentially infected with wild poliovirus is a key component in the decision to stop vaccination. The last smallpox case occurred not in Somalia in 1977, but in England in 1978. The virus was transmitted through a faulty ventilation system from a laboratory to a nearby office, where it infected a person. Like smallpox virus, the only remaining sources of wild virus will be in the laboratories once the virus has been eradicated from the natural environment. The reported transmission of wild poliovirus from a vaccine production facility, presumably through an infected worker, to the community underscores the need for increased containment once wild poliovirus has been eradicated (1).

Infectious or potentially infectious poliovirus materials may be present in a wide range of laboratories, including clinical diagnostic, environmental, research, and teaching. The types of materials that might contain wild poliovirus include clinical (e.g., diagnostic specimens or unidentified enteroviruslike isolates), research (e.g., wild poliovirus strains or derivatives, full-length poliovirus RNA, or cDNA containing full capsid sequences), and environmental (sewage). Because of its high rate of subclinical infections, poliovirus may be found in fecal specimens collected for other purposes. For example, fecal or sewage samples collected in polio-endemic countries for nutritional and environmental studies or studies of other viral, bacterial, or parasitic diseases may contain wild poliovirus.

The WHO Global Action Plan for Laboratory Containment of Wild Polioviruses (2) was published in 1999. The plan is linked to the eradication progress. In the preeradication phase (the present), laboratories are required to implement safe handling procedures for materials infected or potentially infected with poliovirus (biosafety level [BSL] 2/polio). Countries must establish national inventories of laboratories holding such materials. Completion of preeradication activities is required before a region can be certified polio-free. The posteradication phase (high containment) begins 1 year after identification of the world's last case. At that time, laboratories holding wild poliovirus stocks and potentially infectious materials must either place all materials under appropriate biosafety conditions, transfer important virus isolates to WHO interim repositories, or render all wild poliovirus materials noninfectious. Documentation of containment compliance by all regions is required for global certification of poliovirus eradication. For countries that intend to stop all poliovirus vaccination, work with materials that could cause infection with wild poliovirus must be conducted under BSL 4 containment. High containment (BSL 3/polio) will be required for work with vaccine-derived viruses.

High-level political involvement and multi-sector commitments, including departments of health, defense, education, environment, and private industry are essential to achieving and maintaining global containment of wild poliovirus.