[Immunity to diphtheria in the Croatian population in 1994]

The large outbreak of diphtheria in the former SSSR countries, which has been linked, among others, to low herd immunity, has led us to determine the level of this immunity in the Croatian population in view to undertake, if necessary, extra prophylactic measures (vaccination). For the purpose, C. diphtheriae serum antibody levels were determined in persons aged from 20 to above 60 years and coming from four major Croatian towns (Zagreb, Rijeka, Osijek and Split). A commercial ELISA test and neutralization test (NT) on guinea pig were used. Whereas 223 sera were examined with ELISA, only 51 underwent NT. Both tests showed herd immunity to decline gradually with age, ELISA also revealing a wide variation range of antibody concentrations by individual age groups (Table 1). There was not a single serum sample below the "critical" level of 0.01 IU/ml. in Table 2, ELISA results are classified in three groups by antibody level: up to 0.01 IU/ml, between 0.01 and 0.09 and above 0.09 IU/ml, i.e. as unprotected, partially protected and protected persons. Such grouping of findings shows that not one of the subjects belongs to the first category, that most of them belong to the third (68%), as well as that the latter are passing into the second with growing age. It is concluded that by carrying out the mandatory immunization programme, in force in Croatia, it has been possible to attain and to sustain a high level of herd immunity. Hence there is no need for booster immunizations at older age.