The effective reproduction number, also called the effective reproductive number, and sometimes denoted by Re or simply R, estimates or models the average number of infections caused by a case of an infectious disease in a population that includes not only susceptible people. Rt is a variation of this concept that captures variation over time.
For any infectious disease which causes immunity or for which there is a vaccine, a large percentage of the population will not be susceptible, therefore an effective reproduction number allows insights into the intensity of outbreaks in real world populations. If at a given time the effective reproduction number falls below one, it suggests an outbreak may be ending.
The effective reproduction number (R) will vary based on not only on changes in human behavior but also characteristics of a disease, such as how long the disease typically lasts before recovery or death and therefore variation of R over time involves various statistical complexities.
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