Basic reproduction number
Say it: BAY-sik ree-pruh-DUK-shun NUM-bur
The basic reproduction number, R0, is the average number of people one infectious person would infect in a population with no immunity.
Also: R0, R naught, basic reproductive number, basic reproduction ratio
Definition
What is the basic reproduction number?
The basic reproduction number, written R0 and read R-naught, describes how contagious an infection is. It is the average number of new infections that one infectious person would cause in a population where everyone is susceptible and no control measures are in place. The idea came from demography, where it counted offspring, and entered epidemiology in the 1950s.
R0 works as a threshold. If it is above 1, each case leads on average to more than one new case and an outbreak can grow; if it is below 1, chains of transmission die out. R0 depends on three main things: how long a person stays infectious, how likely each contact is to pass on the infection, and how often people make contact. Because contact patterns differ between places and change over time, R0 is not a fixed biological constant of a pathogen, and it says nothing about how severe a disease is. Measles is among the most contagious infections, with an R0 often quoted as 12 to 18.
Once some people are immune or control measures are in place, epidemiologists track the effective reproduction number, R or Rt, instead. R0 also sets the herd immunity threshold, roughly 1 minus 1/R0: for measles, more than 90 percent of a population must be immune to halt spread.
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Terms that go with it
- Herd immunityHerd immunity is the indirect protection a population gains when so many people are immune that an infection can no longer spread easily.
- EpidemicAn epidemic is a rise in cases of a disease, often sudden, above the level normally expected in a given population, area and time.
- PandemicA pandemic is an epidemic that crosses international borders and spreads over several continents, typically infecting very many people.
- VaccineA vaccine is a preparation that trains the immune system to recognize a pathogen, so the body can fight it off quickly if it meets the real thing later.
- EpidemiologyEpidemiology is the study of how often diseases and other health events occur in populations, what causes them and how they can be controlled.
- PathogenA pathogen is a microbe or other agent, such as a virus, bacterium, fungus, parasite or prion, that can cause disease in a host.
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Written for biofideX in plain words, after checking these references:
- CDC Emerging Infectious Diseases: Complexity of the Basic Reproduction Number (R0)
- History of Vaccines (College of Physicians of Philadelphia): How Herd Immunity Works
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