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

Abstract

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.

PIP: Throat diphtheria is uncommon in Africa, with only sporadic cases seen even before the application of vaccination. Outbreaks are rarely reported from Africa. Immunity to the disease is high in subtropical regions due to the frequent presence of Corynebacterium diphtheriae in skin ulcers, while asymptomatic throat carriers also contribute to the spread of the organism. An outbreak of diphtheria occurred in the first 6 months of 1989 in the Quthing district of Lesotho, with 68 clinical cases identified in patients aged 14 months to 51 years. 38% of cases were among people aged 10-15 years, and 26% were among 15-20 year olds. Age-adjusted incidence rates were higher in patients aged 15-35 years than in those aged 0-15 years. The overall case fatality rate was 23%, with most deaths occurring among people aged 10-14 years. This epidemic occurred despite increasing immunization coverage rates through the ongoing vaccination program first launched in Lesotho in 1977. In Quthing district, and since 1984, more than 90% of under-5-year-olds have routinely received 3 doses of DPT vaccine. The authors can only speculate upon the origin and spread of the epidemic in Lesotho because of the absence of reliable data on carrier rates and overall immunity. Nonetheless, since vaccination prevents cases of diphtheria, there are fewer carriers to maintain a certain level of immunity. Certain population groups, such as alcoholics and those born before the introduction of vaccination, may be more vulnerable to diphtheria. Incomplete immunization of a population may therefore lead to herd susceptibility rather than herd immunity. The massive vaccination campaign which began during the outbreak and continued until July 1989 appears to have helped curtail the epidemic.