R0Reff

R0Reff is an informal shorthand sometimes used in discussions of infectious disease dynamics to refer collectively to the basic reproduction number (R0) and the effective reproduction number (Reff or Rt). The two related concepts describe different aspects of pathogen transmissibility and are central to epidemic modeling and public‑health decision making.

R0 (basic reproduction number) estimates the average number of secondary cases generated by a single infectious individual in a completely susceptible population, absent control measures. It is a theoretical property determined by pathogen biology, contact patterns, and mixing structure; values greater than 1 indicate the potential for exponential spread in an unmitigated setting.

Reff (effective reproduction number), often written Re, Rt, or Reff(t), describes the average number of secondary cases at a particular time, accounting for population immunity, behavioral changes, interventions, and other time‑varying factors. In simple compartmental models, Reff ≈ R0 × S, where S is the susceptible fraction; in practice, Reff is influenced by vaccination, prior infection, nonpharmaceutical interventions, and heterogeneity.

Estimation of R0 and Reff uses epidemiological data (case counts, hospitalizations), contact surveys, serial-interval distributions, phylodynamic methods, and statistical or Bayesian inference applied to mechanistic models. Public health use focuses on Reff: values below 1 imply declining transmission, guiding relaxation or intensification of control measures.

Limitations include sensitivity to data quality, changing testing practices, population heterogeneity, waning immunity, and pathogen evolution. Precise interpretation requires attention to model assumptions, temporal and geographic context, and the specific definitions employed by investigators.