The Influence of Time-Limited Immunity on a COVID-19 Epidemic: A Simulation Study

A series of spreadsheet simulations using SEIS, SEIR, and SEIRS models showed that different durations of effective immunity could have important consequences for the prevalence of an epidemic disease with COVID-19 characteristics. Immunity that lasted four weeks, twelve weeks, six months, one year, and two years was tested with pathogen R₀ values of 1.5, 2.3, and 3.0. Shorter durations of immunity resulted in oscillations in disease prevalence. Immunity that lasted from three months to two years produced recurrent disease outbreaks triggered by the expiration of immunity. If immunity “faded out” gradually instead of persisting at full effectiveness to the end of the immune period, the recurrent outbreaks became more frequent. The duration of effective immunity is an important consideration in the epidemiology of a disease like COVID-19.