SEIR (Susceptible-Exposed-Infectious-Recovered) is a compartmental model used to study how infectious diseases, such as COVID-19, spread using ordinary differential equations (ODEs). The model is improved by considering the effects of vaccination and reinfection. The infectious population is further divided into two types: (1) those with moderate to severe symptoms, which can lead to detection and are highly infectious, and (2) those with mild or no symptoms (asymptomatic), which remain undetected and may be less infectious.
References: SSRN, EClinicalMedicine