It was researched and adopted a method to introduce a seasonal behavior on SIR model to study the dynamics of covid-19. This method is based on the calculation of β for each week of the year based on observed previous seasonal behavior for several countries and regions, which are the most affected in the world. Was also included in the model the vaccination, which will be a factor of major effect on this dynamic in 2021. The model was used to build a simulator and was done the determination of β and the forecast of covid-19 cases for USA, Brazil and India. β was found to range seasonally from 0,15 to 0,40 or from 0,10 to 0,80 depending on the region. It was found that vaccination will be very effective in reducing the cases in 2021 and that the herd immunity will be reached when around 55% of the population be immune. The simulation took to some unexpected findings, like the effect of lockdown in later waves of the epidemic and about the epidemic dynamics. It was found a condition for exogenic respiratory viruses that triggers a major epidemic and a condition that explains why a respiratory virus for which part of the population is already immune has a seasonal behavior, with a small number of cases. This dynamic explains the evolution of covid-19 in 2020 and 2021 and even the Spanish flu in 1918 and 1919.