FALL-OFF IN IMMUNITY FOLLOWING DIPHTHERIA REVACCINATION - AN 8 YEAR FOLLOW-UP-STUDY

Diphtheria may occur even among previously vaccinated persons and know ledge of the duration of immunity is of crucial importance when design ing effective vaccination programmes. In a follow-up study of 42 repre sentative probands revaccinated 8 years previously, a continuous fall- off in antitoxic immunity was demonstrated. 98% were still protected ( antitoxin concentration >0.01 IU/ml). From the distribution of titres in the group the individual risk of susceptibility 8 years after revac cination was calculated to be 0.8 parts per thousand (0.2-2.9 parts pe r thousand, 95% confidence limits). Thus, repeated revaccinations are required to secure continuous protection. The fall-off pattern for dip htheria antitoxin was approximately the same as for tetanus antitoxin. Peak values following revaccination are decisive for the duration of immunity. As peak values following vaccination depend on naturally acq uired immunity and consequently decrease as indigenous diphtheria in a population disappears, highly potent vaccines are required to secure long-term immunity following diphtheria revaccination. The effects of dose and adjuvant are discussed.