Immunity to diphtheria in a sample of the Canadian adult population

To assess immunity to diphtheria in a sample of Canadian adults.

A seroprevalence study of a group of plasmapheresis donors was performed over a four-month period in 1996. A convenience sample of 1619 sera was collected to obtain a good distribution by age groups and centres. The determination of diphtheria antitoxin concentrations was performed by neutralization of diphtheria toxin in cell culture.

A total of 1619 plasmapheresis donors from Halifax, Quebec City, London, Calgary and Edmonton were studied.

Of the 1619 sera, 20.3% tested showed susceptibility to diphtheria (antitoxin concentration less than 0.01 IU/mL). The proportion of susceptibles increased from 9.5% in subjects 30 to 39 years of age to 36.3% in those 60 years of age or more. The age group 20 to 29 years demonstrated a higher proportion of susceptibles (18.3%) than the next age group (30 to 39 years) in four of the five centres. Significant differences in antibody levels were also observed among the centres. There was no statistically significant difference between sexes.

Overall, detectable antibody and presumably immunity to diphtheria in the present sample of Canadian adults is relatively good. However, reason(s) for the relatively high proportion of susceptibles in those aged 20 to 29 years of age in certain centres, as well as why Canada has not experienced any diphtheria outbreaks in the past 20 years given these susceptibility levels, should be investigated further.