Reproduction Number EXCEL Model of COVID-19 for predictive calculation of nearby trend up to the end of infection

Reproduction Number (RN-)EXCEL Model has been developed on an EXCEL sheet to provide important characteristics of COVID-19 infectious disease for practical use. The model is developed based only on observed data to predict future infection toward herd immunity threshold and until the end stage of the infection. Basic equations are simple and constructed in analogy with neutron multiplication reactions in nuclear reactor. To know the next day infection, we calculate an exponential increase in one day step with a rate obtained from nearby PCR positive infectious numbers, which are daily input in the EXCEL sheet. In a closed community, main players are non-immune holders and immune holders, where total number of immune holders derived from infection and vaccination plays an essential role. In traditional SIR model, infection behavior is characterized by the reproduction rate in differential equation where social actions such as governmental regulations or vaccinations are included as constant breaking term for infection spread. However, in actual situation these terms are time dependent and is difficult to solve by a set of differential equations. In contrast, RN-EXCEL model deals with infection by defining successive reproduction number for each time interval as inchworm that represents a clear physical picture of the virus infectivity. Using this model, a lot of predictions were made for semi-closed communities domestically and world-wide, timely for practical use.