Europa kan öppna för resor mot slutet av sommaren

In epidemiology, the basic reproduction number (sometimes called basic reproductive ratio, or incorrectly basic reproductive rate, and denoted R0, pronounced R nought or R zero) of an infection can be thought of as the expected number of cases directly generated by one case in a population where all individuals are susceptible to infection. The definition describes the state where no other individuals are infected or immunized (naturally or through vaccination). Some definitions, such as that of the Australian Department of Health, add absence of "any deliberate intervention in disease transmission". The basic reproduction number is not to be confused with the effective reproduction number R (or Re), which is the number of cases generated in the current state of a population, which does not have to be the uninfected state. By definition, R0 cannot be modified through vaccination campaigns. Also, it is important to note that R0 is a dimensionless number and not a rate, which would have units of time-1, or units of time like doubling time.R0 is not a biological constant for a pathogen as it is also affected by other factors such as environmental conditions and the behaviour of the infected population. Furthermore R0 values are usually estimated from mathematical models, and the estimated values are dependent on the model used and values of other parameters. Thus values given in the literature only make sense in the given context and it is recommended not to use obsolete values or compare values based on different models. R0 does not by itself give an estimate of how fast an infection spreads in the population. The most important uses of R0 are determining if an emerging infectious disease can spread in a population and determining what proportion of the population should be immunized through vaccination to eradicate a disease. In commonly used infection models, when R0 > 1 the infection will be able to start spreading in a population, but not if R0 < 1. Generally, the larger the value of R0, the harder it is to control the epidemic. For simple models, the proportion of the population that needs to be effectively immunized (meaning not susceptible to infection) to prevent sustained spread of the infection has to be larger than 1 − 1/R0. Conversely, the proportion of the population that remains susceptible to infection in the endemic equilibrium is 1/R0. 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.