Herd immunity has to do with the protection of populations from infection which is brought about by the presence of immune individuals. The concept has a special aura, in its implication of an extension of the protection imparted by an immunization program beyond vaccinated to unvaccinated individuals and in its apparent provision of a means to eliminate totally some infectious diseases. It is a recurrent theme in the medical literature and has been discussed frequently during the past decade. This new popularity comes as a consequence of several recent major achievements of vaccination programs, i.e.: the historic success of the global smallpox eradication program; dramatic increases in vaccination coverage stimulated by national programs and by the Expanded Programme on Immunization; the commitment of several countries to eradicate measles; and international dedication to eliminate neonatal tetanus and to eradicate poliomyelitis from the world by the year 2000.'

Herd Immunity: History, Theory, Practice

The strategy currently used to control measles in most countries has been to immunize each successive birth cohort through the routine health services delivery system. While measles vaccine coverage has increased markedly, significant measles outbreaks have continued to recur. During the past 5 years, experience in the Americas suggests that measles transmission has been interrupted in a number of countries (Cuba, Chile, and countries in the English-speaking Caribbean and successfully controlled in all remaining countries. Since 1991 these countries have implemented one-time "catch-up" vaccination campaigns (conducted during a short period, usually 1 week to 1 month, and targeting all children 9 months through 14 years of age, regardless of previous vaccination status or measles disease history). These campaigns have been followed by improvements in routine vaccination services and in surveillance systems, so that the progress of the measles elimination efforts can be sustained and monitored. Follow-up mass vaccination campaigns for children younger than 5 years are planned to take place every 3 to 5 years. ( JAMA . 1996;275:224-229)

https://jamanetwork.com/HttpHandlers/ArticlePdfHandler.ashx?journal=JAMA&articleId=394542&pdfFileName=jama_275_3_033.pdf

Measles Elimination in the Americas: Evolving Strategies

https://www.who.int/immunization/newsroom/final_WHO_measles_paper_Lancet.pdf

Has the 2005 measles mortality reduction goal been achieved? A natural history modelling study

Conventional epidemiological models usually assume homogeneous mixing, with susceptible and infected individuals mingling like the molecules in an ideal gas. It has recently been noted, however, that variability in transmission rates—arising, for example, from some hosts being in dense aggregates while others are in small or remote groups—can result in the intrinsic reproductive rate, R0, of a microparasitic infection being greater than would be estimated under the usual assumption of homogeneous mixing; this implies the infection may be harder to eradicate under a homogeneously applied immunization programme (that is, a larger proportion of the population must be vaccinated) than simple estimates might suggest. In this paper we consider a spatially heterogeneous population arbitrarily subdivided into n groups, with one transmission rate among individuals within any one group, and another, lower transmission rate between groups. We define an optimum eradication program as that which—treating different groups differently—achieves its aim by immunizing the smallest overall number in each cohort of newborns, and we show this optimum program requires fewer immunizations than would be estimated under the (false) assumption that the population is homogeneously mixed. We prove this result in general form, and illustrate it for some special examples (in particular, for a population subdivided into one large ”city“ and several small ”villages“).

J.Michael Lane

Papers

The case for global measles eradication.