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.

Impact of vaccination on selected diseases in Canada.

Vaccination has dramatically reduced the morbidity and mortality rates of a number of diseases. The crucial element of vaccination programs is commitment to widespread coverage and to containment of outbreaks. Vaccines have led to virtual elimination of poliomyelitis and promise to eliminate measles. The incidence of congenital rubella syndrome will probably only be diminished if vaccination is extended to all 1-year-olds and susceptible prepubertal girls. The employment of diphtheria toxoid is one of the great success stories in public health. The incidence of pertussis has declined because of the diphtheria-pertussis-tetanus (DPT) vaccine given to infants, although elimination of the disease will probably have to await development of a more potent pertussis antigen. A remarkable reduction in the incidence of tetanus and tuberculosis has also been achieved.

Rubella immunization strategies in Canada.

Rubella vaccine was introduced in Canada in 1969. Immunization practices and vaccine coverage varied from province to province. In the 1970s the Canadian National Advisory Committee on Immunization endorsed both the policy of mass vaccination--in combination with measles and mumps vaccines--for infants, which seven provinces followed, and that of selective immunization of prepubertal girls, which three provinces followed. In 1982, the Committee advocated a comprehensive policy that incorporated the best features of the two policies and also increased the emphasis on immunization of susceptible adolescent and adult women. As of 1983, in all provinces the vaccine has been routinely administered to infants 12-15 months old; in seven, also to prepubertal girls. After the introduction of rubella vaccine, rubella incidence declined markedly, but the endemic level of rubella incidence remained unchanged. Congenital rubella syndrome (CRS) was added to the federal list of notifiable diseases only in 1979. Sixty-seven CRS cases were reported by five provinces from 1979 to 1983, during which a trend of declining CRS incidence rates (per 100,000 live births) was indicated. However, the numbers of cases are too small to draw definite conclusions regarding the impact of immunization programs.

Listeria monocytogenes infections in Canada.

Since its first isolation by Murray in 1926 Listeria monocytogenes has become recognized as a significant pathogen occurring worldwide and involving a wide range of wild and domestic animals including man. The first confirmed human listeriosis case in Canada was published by Stoot in 1951. A later survey based on records maintained at the Laboratory Centre for Disease Control, Ottawa showed 101 cases detected over a 21 year period in nine provinces. The overall mortality was 30%. The most frequently isolated serotype was 4b followed by 1 and 1b. Prior to the Nova Scotia epidemic (41 cases) of 1981, fewer than 15 cases per annum had been diagnosed based on hospital discharge records. The Nova Scotia epidemic was unique in that the source and mode of transmission of the organism were determined. Sixty-three strains isolated from this outbreak were typed, and with the exception of one 1a strain, were identified as 4b. These were subsequently classified mainly as phage type 00 042 0000 and 00 002 0000. Listeriosis appears to be a common infection in the animal population in Canada primarily in cattle, sheep, chinchillas, chickens and goats. Outbreaks have been described in rabbits, goats, and chinchillas. Chinchilla farms were affected in one outbreak (serotype 1) in Nova Scotia which was attributed to feeding a new batch of meal containing beet pulp. Many aspects of the epidemiology of listeriosis are obscure. A cycle involving contaminated soil and consumption of raw vegetables has been confirmed as the cause of the Nova Scotia epidemic and could explain a proportion of the sporadic cases.(ABSTRACT TRUNCATED AT 250 WORDS)

Experience with measles in Canada and the United States.

The United States is attempting to eliminate indigenous measles by October 1982. Progress in this effort appears to be related largely to the fact that all children must now be vaccinated before starting school and that unvaccinated children are not allowed to attend school during epidemics. Canada has not yet made a similar concerted effort to eliminate measles. The epidemiologic features of the disease in Canada and the United states have generally been similar, but some differences have emerged in recent years: Canadian rates are currently about 10 times higher, and the highest incidence in Canada is still in children 5 to 9 years of age, whereas in the United States the highest incidence is now in children aged 10 to 14 years, the result of intensive vaccination programs for preschool and early school-aged children.