Smallpox epidemiology and demographics
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]
Overview
Smallpox was declared eradicated in 1980 by the WHO. The true incidence of smallpox before its eradication is difficult to estimate due to poor reporting from endemic regions, which may have reported only 1-2% of the cases. Children and young adults were the most often affected, especially in regions with low levels of immunity. There is no evidence of gender or race differences in the incidence of the disease. Developing countries had a higher incidence of the disease.[1]
Incidence
The number of new cases, reported to the international health authorities, was often inaccurate. The data obtained from non-endemic countries, with good health services, was probably the most accurate. Yet, according to the Intensified Smallpox Eradication Programme, the reported incidence amounted only to 1-2% of the actual number of cases, which made it impossible to obtain an accurate estimate of the incidence.[1]
In endemic regions there were periods called epidemic years in which the incidence was much higher. In order to try to justify this discrepancy, several possibilities were evoked, such as:[1]
- Viability of the virus
- Changes in susceptibility of the host
- Social factors, such as dispersion of the population
- Seasonal variation in incidence in relation to eradication
Age
The age adjusted incidence of the disease may vary depending upon the level of acquired immunity in the population. When populations were exposed to the disease for the first time, all ages would be affected. In endemic regions, where there was some previous level of immunity, children and young adults were the most severely affected.[2][1][3]
Gender
Smallpox affected males and females equally.[1][3]
Race
The incidence of smallpox did not differ according to the race.[1][3]
Developed Countries
Developed countries, due to a better and established health system, had lower incidence of smallpox and better reports of new cases to international organizations.[1]
Developing Countries
In developing countries where healthcare facilities are sometimes not trusted by the population, cases were sometimes not reported to public health authorities. Also, vaccination was not sanctioned by some religious beliefs. Taken together, these factors might explain at least in part the higher incidence of smallpox in developing countries.[1]