For COVID-19, many of the infected individuals become symptomatic after the latent period is ended and should be isolated from the community. After the isolation period is ended, they become recovered individuals who have immunity and return to the community. Thus, they are capable of infecting susceptible individuals only during the latent period. Some infected individuals, however, are asymptomatic through the recovery period, which includes the latent period. They are infectious but not isolated. They stay in the community and continue infecting susceptible individuals through the recovery period, inducing an increase in the number of infected individuals, though they become recovered individuals who also have immunity in the community after the recovery period is ended. The number of ‘symptomatic and isolated’ infected individuals and/or of ‘asymptomatic and staying’ infected individuals is controlled by the symptomatic rate. Thus, the symptomatic rate affects the number of infectious individuals continuing to infect susceptible individuals in the community. At the same time, the symptomatic rate affects the number of isolated individuals and, as a result, the number of recovered individuals and the population excluding the individuals kept in isolation. Since the contact rate between infected individuals and susceptible individuals is affected by both the number of recovered individuals having returned to the community and the population excluding the individuals kept in isolation, the symptomatic rate also affects the contact rate. Namely, the symptomatic rate affects not only the number of infectious individuals but also the contact rate. This symptomatic rate could change commonly depending on the characteristics of virus and/or on the health conditions of infected individuals. However, the symptomatic rate could be changed/decided in the middle of infection duration by political and/or medical interventions because the symptomatic rate practically means the isolation rate, and the isolation rate and/or the number of isolated individuals could be decided by some political and/or medical interventions induced by, for example, the capacity of hospital care. Therefore, the evaluation of the effect of the symptomatic rate could provide reference materials for political and/or medical measures. The effects of vaccination and isolation by PCR were also examined for cases with different symptomatic rates. The results of evaluation by a flexible compartment model, which is a model including the symptomatic rate as a term in the calculation equation and being able to evaluate the effect of isolated/recovered individuals on the spread of COVID-19, show that a small difference in the symptomatic rate causes a large difference in the number of infected individuals and in the infection duration. Vaccination and PCR tests were effective in reducing the number of infected individuals for cases with any symptomatic rates.