COVID-19(7) Numerical simulation(4) basic reproduction number(4) Reproduction number.(2) existence and uniqueness(2) 92D30(2) Mathematical modelling(2) epidemic model(2) Optimal control(2) Stability analysis.(2) Isolation(2) coronavirus(2) Avian Influenza(2) vaccination.(1) stochastic epidemic model.(1) control strategies(1) Hospitalization(1) epidemic curve(1) different phases of hepatitis B(1) Age-since-infection(1) SIR model(1) optimal control strategy(1) Stochastic model.(1) age structured model(1) asymptomatic transmission(1) 92D40(1) Treatment(1) lockdown(1) 37Fxx(1) prediction(1)