Impact of population density on immunization programmes

Abstract

The eradication of smallpox was achieved by surveillance and containment vaccination after the failure of mass immunization campaigns. The reasons for this failure are considered in this paper. Comparison of population densities in the Indian subcontinent and Africa show that in highly populated areas even an 80% vaccine coverage will still leave a density of susceptibles high enough to maintain the disease, a finding with important implications for other vaccine campaigns.

PIP: The final stages in the successful eradication of smallpox worldwide are interpreted in relation to density of susceptibles. The variola virus led to eradication by an aggressive vaccination campaign followed by surveillance and containment of remaining cases because of several characteristics of the disease: it is transmitted by close face-to-face contact; it has no animal reservoir; the infection does no recur; infectivity does not begin until the rash appears. It was estimated that if vaccination could reach 80% of the population, the disease could be eliminated. This was indeed so in most countries, but not in Nigeria, Bangladesh or India. Data are presented showing that the population density of susceptibles in countries where smallpox disappeared was 1.9 to 3.0/sq km, and the vaccination rates ranged from 80.9% to 84%. In 1973, the average density of susceptibles in Bangladesh was 89/sq km, and in India 35, higher than those the total population density at the time in west or central Africa, except for Nigeria and Gambia. After convincing the health authorities to undertake on active search for cases of smallpox, to isolate the patients and to immunize all contacts, Pakistan recorded its last case of smallpox in 1974, India and Bangladesh in 1975. At that time, the density of susceptibles, related to teh overall vaccination coverage, had not changed from that 3 years ago, when smallpox was still out of control. The relevance of this approach for other diseases such as pertussis, poliomyelitis and measles is discussed.