4.1.1 The eradication of smallpox
On 8 May 1980, the WHO announced that smallpox had been eradicated from the world. Global eradication refers to the permanent worldwide reduction of cases to zero, with no known sources of infection that could generate a new case.
The declaration that the world was free of smallpox came more than two years after the last recorded cases in the three countries where the smallpox virus had proved hardest to eradicate: Somalia, Ethiopia and Kenya.
The global campaign against smallpox began in 1967. Back then, the estimated prevalence of the disease was 10 million cases, it was endemic in more than 30 countries and was frequently imported to at least 12 further countries. Against this background, how was worldwide eradication achieved in little more than ten years? The answer lies in certain features of the smallpox virus, the efficacy of the vaccine that prevents it, and the manifestations of the disease, as explained in Box 3.
Box 3 Special features of smallpox as an eradication target
- Smallpox is a DNA virus with a relatively stable genome, which does not generate variant strains; therefore, the same vaccine could be used everywhere.
- Humans are the only host – there is no reservoir of smallpox virus in other animals or in the natural environment.
- The transmission of the virus from infected to susceptible individuals is relatively infrequent – typically, a person with smallpox infects two to five others (by comparison, a person with measles typically infects 10–20 susceptible contacts), so an outbreak spreads relatively slowly.
- Case finding is relatively easy because symptoms develop soon after infection, the smallpox lesions are immediately visible and characteristic in their appearance (Figure 8), and there is no asymptomatic or carrier state.
- The smallpox vaccine is heat-stable, so it could be transported without loss of efficacy for mass vaccination campaigns in locations where refrigeration would have been impossible.
- A single vaccination produces long-lasting immunity, so there is no need to locate recipients for repeat vaccinations.
A key aspect of the public health approach to smallpox eradication was the adoption of a case containment policy. Instead of removing smallpox patients to hospitals for treatment, where the disease rapidly spread, most infected people were supported to remain in their own homes. Infected villages were sealed off until the patient recovered or died and all known or suspected contacts had been vaccinated.