Perusreproduktiivisnumero

Perusreproduktiivisnumero, known as the basic reproductive number and denoted R0, is an epidemiological measure that expresses the average number of secondary infections produced by a typical infectious person in a completely susceptible population, in the absence of control measures. R0 indicates the pathogen’s transmission potential under ideal conditions; it does not remain fixed across populations or time and is distinct from the effective reproduction number Rt, which decreases as immunity builds or interventions are implemented.

R0 depends on factors including the probability of transmission per contact, the rate of contacts in the population, and the duration of infectiousness. In simple SIR-type models, R0 can be written as R0 = β/γ or, equivalently, R0 = β c D, depending on notation and model structure, where β is transmission probability, c is contact rate, and D is the infectious period.

Estimates vary by pathogen and setting. Measles is among the most transmissible, with R0 often estimated 12–18; influenza estimates typically around 1.3–1.8; SARS-CoV-2 early estimates ranged roughly 2–3, with higher values in some settings and variants.

R0 is used to assess outbreak potential and to determine herd immunity thresholds. The critical vaccination coverage required to prevent sustained transmission is approximately 1 − 1/R0, assuming homogeneous mixing. Limitations include heterogeneity in contact patterns, varying susceptibility, and changes in behavior, immunity, and interventions over time.

See also: reproduction number, Rt, epidemiological modeling.