The aim of this study, carried out in 1993, was to evaluate diphtheria
immunity in Siena.
Diphtheria antitoxin levels were measured by means of the immunoenzymatic
test (ELISA) in
serum samples of 602 apparently healthy subjects (239 males and 363 females)
of all ages
residing in Siena. According to widely used criteria, 6% of the total population
were
susceptible to diphtheria (antibody levels <0·01 IU/ml),
71% had basic protection
(0·01–0·09 IU/ml) and 23% were fully protected
([ges ]0·1 IU/ml). The results suggested that a
high proportion of young population had a protective level of immunity
against
diphtheria, that susceptibility increased with age and a smaller proportion
of males
(2·9%) than females
(8·3%) were unprotected; this difference was statistically significant.
Our results suggest that it
may be useful to revaccinate adults with low levels of diphtheria toxoid
so
that the percentage
that remains unprotected does not put the community at risk of an outbreak
of diphtheria.