Principles of Measles & Rubella Control and Elimination
The crucial factor determining the spread of infections transmitted person-to-person, such as measles and rubella, is the number of secondary cases caused by each infectious person. The basic reproduction number, R 0 , is a measure of the transmissibility of an infection within a population, defined as the average number of secondary infections produced by a typical infective person in a totally susceptible population. It depends on the characteristics of the infectious agent (e.g., infectivity and duration of infectiousness) and of the population (e.g., population density and social mixing patterns). The R0 of measles has been estimated as 14–18 in England and Wales, 12.5 in North America, and 10 in Niger. The R0 of rubella is generally lower (4–7), however in high density settings (e.g., in Addis Ababa, Ethiopia) R0 values as high as 11.8 have been estimated.
In the context of a vaccination programme, the effective reproduction number, R, is key for predicting and preventing outbreaks.R is the average number of infectious individuals resulting from a single infective introduced into the population, given the population mix of vaccine-acquired and naturally acquired immunity at that time, and decreases as population immunity increases. If R is below 1, then the average case will give rise to less than 1 case and transmission will eventually cease. Consequently, elimination programmes aim to keep R below 1. Births and immigration increase population susceptibility (and hence R), while vaccination slows the rate of this increase. High vaccine coverage induces a period of low incidence termed the 'honeymoon period' during which cohorts of susceptible children who were not immunized in the early years of the programme can reach older ages before being exposed. Susceptible persons gradually accumulate until 'post-honeymoon' outbreaks occur. Although the proportion of cases in older children increases after vaccination, the absolute number of such cases may still fall because of the overall reduction in incidence. For rubella, it is crucial to achieve and sustain adequate coverage to avoid an increased incidence in adults, as discussed later in this paper.