Herd Immunity

The Revised Authoritative Guide To Vaccine Legal Exemptions

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Herd immunity is essentially a simple concept describing the totality of naturally acquired and vaccine-based immunity to a given infectious agent as a proportion of the whole population. While the individual objective of vaccination is clearly to prevent or reduce the risk of infection for the individual concerned, the public health objective of vaccination is to increase the level of herd immunity to that required for control or elimination of the infection from the population, and in the longer term on a regional or global scale to eradicate the infection altogether. (Note the distinction between elimination where an infectious agent is no longer present in a particular population and eradication in which the agent is eliminated world- or region-wide; the smallpox virus remains, at the time of writing, the only infection for which global eradication has been achieved.) The term herd immunity is occasionally used to describe the level of population immunity that will result in elimination of a specific infection from a population, but here herd immunity for elimination would be more appropriate. This level of herd immunity for elimination is that at which an infection cannot propagate effectively in the population; at this level of immunity there may be some secondary cases or even short chains of infection, but these chains are sooner or later broken and prove insufficient to prevent the infection from dying out. In other words, the effective reproduction number, R, in these circumstances is less than 1.0. Herd immunity is obviously dynamic, as natural or vaccine-based immunity is lost over time through waning of immunological memory or deaths of immune individuals, and newly susceptible individuals arrive through births or migration. Thus to maintain the effectiveness of vaccination programs, vaccine coverage and herd immunity need to be monitored at regular intervals and if necessary a strategy planned, informed by dynamic modeling, of supplementary vaccination programs introduced to boost herd immunity to the necessary level (Nokes and Anderson, 1988).

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Blood Pressure Health

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