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Diphtheria remains endemic in developing countries, but there are limited published data on the subject. Thailand's diphtheria surveillance data are relatively complete and may give a fuller picture of the situation in similar countries. After routine immunization began in 1977, the incidence of reported diphtheria decreased by >98% to <0.1 case per 100,000 persons annually in the 1990s. Despite infant immunization coverage of >90%, diphtheria cases were reported throughout the 1990s, primarily among children <15 years old. Outbreaks were linked to both persistent endemic circulation and to importation of toxigenic Corynebacterium diphtheriae; suboptimal immunization coverage in minority and disadvantaged groups contributed. A serologic survey found 25% of adults 20-39 years old and 14% of adolescents 10-19 years old lacked immunity to diphtheria; these data indicate an accumulation of susceptible adolescents and adults. Diphtheria remains a threat in Thailand; improvements in diphtheria control will depend on improving childhood immunization coverage in Thailand and the surrounding region.

The diphtheria epidemic in the former Soviet Union reached Azerbaijan in 1991, when 66 cases of diphtheria were reported, a number that compared with 4 cases in 1990. From 1990-1996, 2182 cases of diphtheria and 286 diphtheria fatalities (case fatality rate: 13.1%) were reported in Azerbaijan, primarily among persons 5-39 years of age. Almost 45% of cases and 61% of deaths occurred among children 5-14 years of age. The high burden of severe disease among children and young adults suggested a different pattern of preexisting immunity against diphtheria in the Azerbaijani population than was observed in the concurrent diphtheria epidemic in Russia. Because resources were limited in Azerbaijan, mass immunization of the population was carried out in stages, focusing initially on school-aged children. Mass immunization campaigns targeting children were moderately successful in stabilizing the epidemic; mass immunization campaigns targeting both adults and children were eventually needed to fully stop the epidemic.

A resurgence of diphtheria spread throughout the Russian Federation in the early 1990s; diphtheria had been well controlled, but circulation of toxigenic strains of Corynebacterium diphtheriae had persisted since the implementation of universal childhood vaccination in the late 1950s. More than 115,000 cases and 3,000 deaths were reported from 1990 to 1997, and, in contrast to the situation in the prevaccine era, most of the cases and deaths occurred among adults. Contributing factors included the accumulation of susceptible individuals among both adults and children and probably the introduction of new strains of C. diphtheriae. Vaccine quality, vaccine supply, or access to vaccine providers did not significantly contribute to the epidemic. Mass vaccination of adults and improved childhood immunization controlled the epidemic. High levels of population immunity, especially among children, will be needed to prevent and control similar outbreaks in the future.

The Newly Independent States (NIS) inherited a common approach to diphtheria control from the Soviet Union and maintained a centralized system of surveillance and control managed by Soviet-trained epidemiologists with a shared professional culture. This system had controlled a diphtheria resurgence in the 1980s. In response to the epidemic of the 1990s, NIS health authorities responded with a set of control measures based on the Soviet-era experience. These measures included intensified childhood vaccination, aggressive case investigation, widespread diphtheria screening in institutions, and vaccination of adults in high-risk occupation groups. These measures proved insufficient due to high levels of susceptibility among adults, excessive contraindications to childhood vaccination, and insufficient resources in many countries. After these initial delays in implementing effective measures in some countries, most of the NIS health authorities rapidly and successfully implemented mass immunization of the population against diphtheria once the strategy was adopted and sufficient vaccine was available.

The Republic of Uzbekistan, like the other Newly Independent States in the 1990s, experienced epidemic diphtheria during the 1990s. The outbreak in Uzbekistan began in 1993 in southern regions that bordered areas of Tajikistan that were experiencing a very intense diphtheria epidemic. However, the Uzbek epidemic rapidly spread and threatened to involve the entire country. From 1993-1996, 1169 cases of diphtheria were reported, compared with 58 in 1990-1992. Unvaccinated or only partially vaccinated cases were more likely to have clinically severe forms of diphtheria than those who were fully vaccinated. Strong epidemiologic links with the Tajik diphtheria epidemic and the predominance of mitis biotype strains of Corynebacterium diphtheriae in Uzbekistan make it likely that the Uzbek outbreak arose independently of the predominantly biotype gravis epidemic that began in Russia. The epidemic appeared to be due to low population immunity and the large-scale reintroduction of toxigenic strains of C. diphtheriae. Several mass vaccination campaigns and general enhancement of routine immunization procedures led to control of the epidemic in 1996.

In 1990, epidemic diphtheria reemerged in Russia and spread to Belarus in 1992, when 66 cases were reported. Diphtheria cases doubled each year in 1993 and 1994 and peaked in 1995, when 322 cases were reported. Intensified routine immunization of young children and mass vaccination of older children and selected groups of adults were conducted in 1995 and were followed by mass vaccination campaigns targeting all adults in 1996. By the end of 1996, full immunization of >95% of children and coverage of>87% of adults with >/=1 dose resulted in a rapid decline in diphtheria cases. In 1998, only 36 cases of diphtheria were reported. More than 70% of the 965 cases and 26 fatalities reported during 1990-1998 occurred among persons >14 years of age. High levels of immunity among the entire population are needed for rapid control of diphtheria epidemics in the vaccine era.

The massive reemergence of diphtheria in the Newly Independent States of the former Soviet Union marked the first large-scale diphtheria epidemic in industrialized countries in 3 decades. Factors contributing to the epidemic included a large population of susceptible adults; decreased childhood immunization, which compromised what had been a well-established childhood vaccination program; suboptimal socioeconomic conditions; and high population movement. The role of a change in the predominant circulating strains of Corynebacterium diphtheriae in this epidemic remains uncertain. Massive, well-coordinated international assistance and unprecedented efforts to vaccinate adults were needed to control the epidemic.