Coronavirus Disease 2019 (COVID19) has been a global public health threat. For countries battling the coronavirus pandemic, stringent social or physical distancing (referred to as lockdown), contact tracking in real-time (referred to as trackdown), and/or herd immunity measures have been used to flatten the COVID-19 epidemic curve. This study aimed to examine the effect of lockdown, trackdown, and herd immunity on daily COVID19 cases and related deaths using time series data from Jan 22, 2020 to Nov 05, 2020. The Susceptible- Infectious-Recovered (SIR) model was applied to characterize the epidemic dynamics and to predict possible contagion scenarios of COVID-19. Optimized parameter values were used to estimate the basic reproductive number (R0) for Sweden, South Korea, Germany, and the United States. Our results suggested that lockdown and trackdown measures have successfully controlled the outbreak and transmission of COVID-19, and herd immunity was able to prevent the spread of COVID-19 in Sweden. However, even if the proportion of immunized individuals in the population as a whole may surpass the herd immunity threshold, these pockets of susceptible individuals are still at risk for local outbreaks as immunity is unevenly distributed within a population. This study also found that lockdowns should remain in place for at least 90 days to prevent the epidemic growth, and both social distancing and public health measures could potentially remove the need for the lockdowns. The fewer confirmed cases and deaths were ascertained by trackdowns.