Circulating diphtheria antitoxin levels in children aged 11-14 years in relation to the vaccinal history

Diphtheria antitoxin level in serum samples obtained from 204 healthy children aged 11-14 years was determined by means of an indirect haemagglutination technique and related to the vaccinal history of the subjects. Irrespective of the time since the last toxoid inoculation, the mean antitoxin titre per ml of serum in the individuals who had received incomplete/irregular anti-diphtheria vaccination in childhood was significantly higher when the number of toxoid doses was higher (from two to more than four doses); the same was not observed for individuals given primary vaccination (three toxoid doses) according to the schedule for childhood vaccination in Italy (regular vaccinees) and one or more booster doses. Between 8 and 13 years after the last of three toxoid doses, a significantly negative association between mean antitoxin titre and time was observed only in irregularly vaccinated children (r = -0.82; p less than 0.05); nevertheless, up to thirteen years after the last vaccine dose, more than 95% of the children exhibited protective levels of antitoxin (greater than 0.125 turkey red blood cells passive haemagglutination units per ml). No significant decrease in mean antitoxin titre was observed between 4 and 10 years after the last of four either regularly or irregularly administered toxoid doses.