Epidemics of Poliomyelitis in Africa since 1993

To the Editor—Chumakov and Ehrenfeld [1] have described how cases of vaccine-associated paralytic poliomyelitis and the discovery of circulating vaccine-derived poliovirus have complicated efforts to eliminate poliovirus by vaccination. They do not, however, mention that serious epidemics of poliomyelitis have occurred in Africa since 1993, after the World Health Organization's decision in 1988 to eradicate the disease by means of the live attenuated oral poliovirus vaccine. Before this campaign started, very effective naturally acquired immunity developed if the first contact with poliovirus occurred during infancy [2]. However, infection has reappeared in countries in which the wild virus was removed by vaccination, and first contact now occurs during childhood and adolescence. As a result, epidemics have occurred in Angola [3], Namibia (twice) [4, 5], the Democratic Republic of the Congo [6], and Cape Verde [7], with older children and some adults being affected with respiratory paralysis at a very high mortality rate. In Angola, 1000 cases and >80 polio-related deaths occurred [8]. In the 2006 epidemic in Namibia, the mortality rate was 31% [5]; in Cape Verde, it was 57% among those >15 years old [5]. These epidemics are, therefore, very similar to the so-called virgin-soil epidemics described by Burnet and White [9], which occurred in such isolated regions as St. Helena, Guam, New Guinea, Samoa, and arctic Hudson Bay. In these places, the population had not been exposed to poliovirus early during life and, hence, had not acquired immunity. These epidemics were characterized by high mortality, with adults being affected with paralysis at a high frequency. Among Eskimos, the paralysis rate was 40%, but no cases of paralysis occurred in infants <3 years of age. The mortality rate was 14%. Despite warnings about the dangers of the current strategy [10, 11] and despite references to the classic description of the natural history of poliomyelitis by Burnet [10, 12], the latest epidemic in Namibia has caught the World Health Organization completely by surprise [13]. According to Aylward, outbreaks among adults are “as rare as hen's teeth” [13, p. 1581]. Neither have those who use mathematical models of polio transmission predicted that serious epidemics in Africa could occur. Indeed, it has been stated that “programmes of mass vaccination against poliomyelitis in developing countries are unlikely to do harm by increasing the net incidence of paralytic complications at intermediate levels of coverage” [14, p. 116]. In contrast, in a “mild exercise in fantasy,” Burnet predicted that “if we were to take a whole community of people of all ages, none of whom had ever met the virus of poliomyelitis, and in some way arrange that they were all infected with a single type of virus, we should find that only some of them became paralysed. The highest proportion both of paralysis and deaths would probably be in persons between the ages of fifteen and twenty-five, the smallest would be in infants” [9]. In Namibia, male young adults were predominantly affected [5]. Sadly, the exercise in fantasy has become a reality. Potential conflicts of interest. C.L.C.: no conflicts.