Frequent Links
Basic reproduction number
Disease  Transmission  R_{0} 

Measles  Airborne  12–18 
Pertussis  Airborne droplet  12–17 
Diphtheria  Saliva  6–7 
Smallpox  Airborne droplet  5–7 
Polio  Fecaloral route  5–7 
Rubella  Airborne droplet  5–7 
Mumps  Airborne droplet  4–7 
HIV/AIDS  Sexual contact  2–5 
SARS  Airborne droplet  2–5^{[2]} 
Influenza (1918 pandemic strain) 
Airborne droplet  2–3^{[3]} 
Ebola (2014 Ebola outbreak) 
Bodily fluids  1.52.5^{[4]} 
In epidemiology, the basic reproduction number (sometimes called basic reproductive ratio, or incorrectly basic reproductive rate, and denoted R_{0}, r nought) of an infection can be thought of as the number of cases one case generates on average over the course of its infectious period, in an otherwise uninfected population.^{[5]}
This metric is useful because it helps determine whether or not an infectious disease can spread through a population. The roots of the basic reproduction concept can be traced through the work of Alfred Lotka, Ronald Ross, and others, but its first modern application in epidemiology was by George MacDonald in 1952, who constructed population models of the spread of malaria.
When
 R_{0} < 1
the infection will die out in the long run. But if
 R_{0} > 1
the infection will be able to spread in a population.
Generally, the larger the value of R_{0}, the harder it is to control the epidemic. For simple models and a 100%effective vaccine, the proportion of the population that needs to be vaccinated to prevent sustained spread of the infection is given by 1 − 1/R_{0}. The basic reproduction number is affected by several factors including the duration of infectivity of affected patients, the infectiousness of the organism, and the number of susceptible people in the population that the affected patients are in contact with.
In populations that are not homogeneous, the definition of R_{0} is more subtle. The definition must account for the fact that a typical infected individual may not be an average individual. As an extreme example, consider a population in which a small portion of the individuals mix fully with one another while the remaining individuals are all isolated. A disease may be able to spread in the fully mixed portion even though a randomly selected individual would lead to fewer than one secondary case. This is because the typical infected individual is in the fully mixed portion and thus is able to successfully cause infections. In general, if the individuals who become infected early in an epidemic may be more (or less) likely to transmit than a randomly chosen individual late in the epidemic, then our computation of R_{0} must account for this tendency. An appropriate definition for R_{0} in this case is "the expected number of secondary cases produced by a typical infected individual early in an epidemic".^{[6]}
Contents
Other uses
R_{0} is also used as a measure of individual reproductive success in population ecology,^{[7]} evolutionary invasion analysis and life history theory. It represents the average number of offspring produced over the lifetime of an individual (under ideal conditions).
For simple population models, R_{0} can be calculated, provided an explicit decay rate (or "death rate") is given. In this case, the reciprocal of the decay rate (usually 1/d) gives the average lifetime of an individual. When multiplied by the average number of offspring per individual per timestep (the "birth rate" b), this gives R_{0} = b / d. For more complicated models that have variable growth rates (e.g. because of selflimitation or dependence on food densities), the maximum growth rate should be used.
Limitations of R_{0}
When calculated from mathematical models, particularly ordinary differential equations, what is often claimed to be R_{0} is, in fact, simply a threshold, not the average number of secondary infections. There are many methods used to derive such a threshold from a mathematical model, but few of them always give the true value of R_{0}. This is particularly problematic if there are intermediate vectors between hosts, such as malaria.
What these thresholds will do is determine whether a disease will die out (if R_{0} < 1) or whether it may become epidemic (if R_{0} > 1), but they generally can not compare different diseases. Therefore, the values from the table above should be used with caution, especially if the values were calculated from mathematical models.
Methods include the survival function, rearranging the largest eigenvalue of the Jacobian matrix, the nextgeneration method,^{[8]} calculations from the intrinsic growth rate,^{[9]} existence of the endemic equilibrium, the number of susceptibles at the endemic equilibrium, the average age of infection ^{[10]} and the final size equation. Few of these methods agree with one another, even when starting with the same system of differential equations. Even fewer actually calculate the average number of secondary infections. Since R_{0} is rarely observed in the field and is usually calculated via a mathematical model, this severely limits its usefulness.^{[11]}
In popular culture
In the 2011 film Contagion, a fictional medical disaster thriller, R_{0} calculations are presented to reflect the progression of a fatal viral infection from case studies to a pandemic.
Further reading
 Jones, James Holland. "Notes on R0" (PDF). Retrieved 15 September 2014.
See also
 Eepidemiology
 Epi Info software program
 Epidemic model
 Epidemiological methods
 Epidemiological Transition
References
 ^ Unless noted R_{0} values are from: History and Epidemiology of Global Smallpox Eradication From the training course titled "Smallpox: Disease, Prevention, and Intervention". The CDC and the World Health Organization. Slide 1617.
 ^ Wallinga J, Teunis P (2004). "Different epidemic curves for severe acute respiratory syndrome reveal similar impacts of control measures". Am. J. Epidemiol. 160 (6): 509–16. PMID 15353409. doi:10.1093/aje/kwh255.
 ^ Mills CE, Robins JM, Lipsitch M (2004). "Transmissibility of 1918 pandemic influenza" (PDF). Nature 432 (7019): 904–6. PMID 15602562. doi:10.1038/nature03063.
 ^ Althaus, Christian L. (2014). "Estimating the Reproduction Number of Ebola Virus (EBOV) During the 2014 Outbreak in West Africa". PLoS Currents. doi:10.1371/currents.outbreaks.91afb5e0f279e7f29e7056095255b288.
 ^ Christophe Fraser; Christl A. Donnelly, Simon Cauchemez et al. (19 June 2009). "Pandemic Potential of a Strain of Influenza A (H1N1): Early Findings". Science 324 (5934): 1557–1561. PMC 3735127. PMID 19433588. doi:10.1126/science.1176062. Free text
 ^ O Diekmann; J.A.P. Heesterbeek and J.A.J. Metz (1990). "On the definition and the computation of the basic reproduction ratio R_{0} in models for infectious diseases in heterogeneous populations". Journal of Mathematical Biology 28 (4): 356–382. PMID 2117040. doi:10.1007/BF00178324.
 ^ de Boer, Rob J. Theoretical Biology (PDF). Retrieved 20071113.
 ^ Diekmann O and Heesterbeek JAP (2000). Mathematical epidemiology of infectious diseases: model building, analysis and interpretation. New York: Wiley.
 ^ Chowell G, Hengartnerb NW, CastilloChaveza C, Fenimorea PW and Hyman JM (2004). "The basic reproductive number of Ebola and the effects of public health measures: the cases of Congo and Uganda". Journal of Theoretical Biology 229 (1): 119–126. PMID 15178190. doi:10.1016/j.jtbi.2004.03.006.
 ^ Ajelli M, Iannelli M, Manfredi P and Ciofi degli Atti, ML (2008). "Basic mathematical models for the temporal dynamics of HAV in mediumendemicity Italian areas". Vaccine 26 (13): 1697–1707. PMID 18314231. doi:10.1016/j.vaccine.2007.12.058.
 ^ Heffernan JM, Smith RJ, Wahl LM (2005). "Perspectives on the Basic Reproductive Ratio" (PDF). Journal of the Royal Society Interface 2 (4): 281–93. PMC 1578275. PMID 16849186. doi:10.1098/rsif.2005.0042.


