Immunity to diphtheria in the 3-19 year age group in Italy.

In Italy, immunization with diphtheria toxoid has been compulsory for all newborns since 1939. The last two clinical cases of diphtheria were reported in 1987. During the period 1987-1989, immunity against diphtheria was assessed by neutralization test in a random sample of 1740 healthy subjects 3-19 years old, from five geographical areas of Italy. Of the total population, 76.5% showed antibody levels considered to be protective (greater than or equal to 0.1 IU ml-1), 17.2% had a relative degree of protection (0.01-0.09 IU ml-1), and 6.3% lacked immunity (less than 0.01 IU ml-1). The percentage of unprotected subjects increased from 6.1% in the age group of 3-5 years to 11.4% in the age group of 18-19 years (p less than 0.01). A smaller proportion of males (5.3%) than of females (7.2%) was unprotected, but this difference was not statistically significant. Subjects residing in the south and the islands were more likely to be unprotected than those residing in the north (7.4 versus 4.1%, p less than 0.01). No association was found between lack of protective antibodies and family size (odds ratio 1.35, confidence interval 95% = 0.77-2.36). However, paternal education of less than 12 years was associated with a higher prevalence of non-responders. In order to maintain a high degree of immunity in the adult population, a routine adult booster dose of diphtheria toxoid is advisable.