A diphtheria epidemic in Lesotho, 1989. Did vaccination increase the population's susceptibility?

We report a diphtheria outbreak which occurred in the 1st 6 months of 1989 in the Quthing district of Lesotho in Southern Africa. Sixty-eight clinical cases were identified in patients with ages ranging from 14 months to 51 years. The 10-15-year age group represented 38% of the cases and the 15-20-year age group another 26%. Age-adjusted incidence rates were higher in patients aged between 15 and 35 than in those aged 0-15 years (p < 0.001). The overall case fatality rate was 23%. Most deaths occurred in the age range 10-14 years, but the case fatality rate tended to be higher in the younger age groups. The epidemic raises the question of the influence of the ongoing vaccination programme, established since 1977, and the importance of naturally acquired immunity. It is suggested that the number of throat carriers, who in the absence of skin diphtheria would provide most of the population's immunity, decreases as an indirect consequence of vaccination, and that certain groups not immediately reached by vaccination become more susceptible to diphtheria. This outbreak also illustrates that a sporadic case of diphtheria in a partially immunized community warrants serious efforts to curb the spread of the disease.