A mathematical model of COVID-19 with minimal compartments is developed. The model is simple enough to fit data on confirmed cases, estimate the hidden infection figure and incorporate the effect of vaccination. With the effect of the new variant being considered, the model fits well with Philippine data of confirmed cases. With the fitted parameters, results show that a second peak, strikingly similar in magnitude to the first, is to be expected if the current transmission rate is increased by about 5\% and already a complete disaster if it will go up to 20\%. The model is then used to estimate time to achieve herd immunity considering natural immunity from being infected and vaccination plan of the country. Results show that if the plan of 200,000 vaccinated people per day is achieved then herd immunity can be attained by March 2022. But if only half of that is achieved the time could go up to February 2023.
Effect of Vaccination to COVID-19 Disease Progression