Mar 13 2020
The basic reproduction number (R0), pronounced “R naught,” is intended to be an indicator of the contagiousness or transmissibility of infectious and parasitic agents.
Myth: R0 is a single numeric value or low–high range, and the interpretation is straightforward; an outbreak is expected to continue if R0 has a value >1 and to end if R0 is <1. The potential size of an outbreak or epidemic often is based on the magnitude of the R0 value for that event, and R0 can be used to estimate the proportion of the population that must be vaccinated to eliminate an infection from that population. This conclusion is sometimes used to suggest that an aim of vaccination campaigns is to remove susceptible members of the population to reduce the R0 for the event to <1.
Facts:
The process of defining, calculating, interpreting, and applying R0 is far from straightforward. The simplicity of an R0 value and its corresponding interpretation in relation to infectious disease dynamics masks the complicated nature of this metric. Although R0 is a biological reality, this value is usually estimated with complex mathematical models developed using various sets of assumptions. Because many researchers using R0 have not been trained in sophisticated mathematical techniques, R0 is easily subject to misrepresentation, misinterpretation, and misapplication.
For any given infectious agent, the scientific literature might present numerous different R0 values. Estimations of the R0 value are often calculated as a function of 3 primary parameters—the duration of contagiousness after a person becomes infected, the likelihood of infection per contact between a susceptible person and an infectious person or vector, and the contact rate—along with additional parameters that can be added to describe more complex cycles of transmission.
Further, the epidemiological triad (agent, host, and environmental factors) sometimes provides inspiration for adding parameters related to the availability of public health resources, the policy environment, various aspects of the built environment, and other factors that influence transmission dynamics and, thus, are relevant for the estimation of R0 values.
Any factor having the potential to influence the contact rate, including population density (e.g., rural vs. urban), social organization (e.g., integrated vs. segregated), and seasonality (e.g., wet vs. rainy season for vector-borne infections), will ultimately affect R0.
the value of R0 is a function of human social behavior and organization, as well as biological characteristics of particular pathogens. More than 20 different R0 values (range 5.4–18) were reported for measles in a variety of study areas and periods, and a review in 2017 identified feasible measles R0 values of 3.7–203.3. This wide range highlights the potential variability in the value of R0 for an infectious disease event on the basis of local sociobehavioral and environmental circumstances.
Vaccination campaigns reduce the proportion of a population at risk for infection and have proven to be highly effective in mitigating future outbreaks. Although the removal of susceptible members from the population will affect infection transmission by reducing the number of effective contacts between infectious and susceptible persons, this activity will technically not reduce the R0 value because the definition of R0 includes the assumption of a completely susceptible population. When examining the effect of vaccination, the more appropriate metric to use is the effective reproduction number (R), which is similar to R0 but does not assume complete susceptibility of the population and, therefore, can be estimated with populations having immune members.
Adopted from https://wwwnc.cdc.gov/eid/article/25/1/17-1901_article