Eradication of indigenous poliomyelitis in Canada: impact of immunization strategies.

During the period 1950-1954, surveillance for paralytic poliomyelitis in Canada revealed an average of 1,914 cases (13.2 cases per 100,000) annually. The licensing and widespread use of inactivated poliovirus vaccine (IPV) in 1955 coincided with a marked decline in disease rates. Due to incomplete vaccine coverage of the population, a resurgence began in 1958 and peaked in 1959, despite an observed vaccine efficacy of 96% for 3 doses of IPV. The introduction and widespread use of oral poliovirus vaccine (OPV) started in 1960 and coincided with a decline in disease rates. Virtual elimination of the natural disease was achieved in the 1970s in all provinces regardless of the specific immunization program chosen (IPV or OPV alone or combined). From 1965 to 1988, 51 cases of paralytic poliomyelitis were reported in Canada. Thirty-five of these cases, all but one occurring before 1980, were attributed to wild virus infection, (14 caused by imported virus and 21 assumed to be endemic). Sixteen cases were OPV-associated: 4 in vaccine recipients and 12 in contacts of OPV recipients. Vaccine-associated paralysis in recipients and contacts occurred at the rate of one case per 9.5 million and 3.2 million vaccine doses distributed, respectively. The risk of paralysis attributable to OPV therefore is small compared to the overall benefit of the vaccine. Both IPV and OPV appear equally effective, and theoretically, a combination of the two (IPV followed by OPV) provides the best risk benefit ratio. Occasional exposure of the Canadian population to imported wild virus requires that high levels of population immunity be maintained.