Why Do Epidemics Evolve in Waves?

Multiple epidemic waves have been observed during the “Spanish Flu” (1918-1919) and the Covid-19 (2019-2021) pandemics. The “Spanish Flu” pandemic, characterized by the H1N1 viral strain, was a severe and well documented pandemic that manifested itself in three distinct epidemic waves spanning a period of 18 months. The Covid-19 pandemic has also been characterized by multiple epidemic waves and the its data also include information on the variants of the ancestor virus. In this study, potential factors contributing to the occurrence of multiple waves are discussed by employing the Susceptible- Infected- Removed SIR model. These factors may include seasonality effects and relaxation of control measures. The introduction of a new variant of a pathogen can initiate a new wave, representing a distinct epidemic event. Also, at the end of an epidemic wave, the relaxation of restrictions allows previously protected individuals to re-enter the susceptible population, leading to an increase in susceptible individuals (S). Finally, the easing of restrictions promotes higher interconnections within the susceptible population, resulting in an increase in the basic reproduction number (R_0). By observing the simulations using the Susceptible-Infected-Removed model, we can conclude that the introduction of a new variant seems to be more dominant among causes leading to a new wave.