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Coronavirus disease 2019 (COVID-19) has dominated almost every aspect of human life on planet Earth since it was first discovered in Wuhan, Hubei province, China, in December 20191. A quick google search of the expression COVID-19 reveals more than 3.22 billion results.
In order to understand the importance of this type of virus, we need to carry out two extremely important epidemiological issues: the basic reproduction number (Ro) and the effective infection number (R). Ro is used to measure the transmission potential of a virus. This number is an average of how many people to whom an infected patient is able to transmit the pathogen, assuming that people close to the patient are not immune to it. Now let's understand R. A population will rarely be completely susceptible to infection in the real world. Some contacts will be immune due to a previous infection that conferred immunity or as a result of previous immunization, due to the action of vaccines. Therefore, not all contacts will be infected and the average number of secondary cases per infectious case will be less than Ro. In this calculation, we take into account susceptible and non-susceptible people. With this information presented, we can conclude that if the value of R is >1, the number of cases will increase, starting an epidemic. For a virus to stop spreading, R must be <1. To estimate R, we multiply the value of Ro by the susceptible fraction of a population2.
