Table 4.

Expanded compartments that considered both virus characteristics and interventions

Interventions	Expansion compartments	Interpretation	References
Social distancing, wearing masks, washing hands, etc	U	Unsusceptible a(U)	[43]
Quarantined at home/hospitalization	ST	Susceptible persons removed from isolation b(ST)	[44]
	I2	Infectious after receiving ineffective treatment (I2)	[45]
	Q, D	Home quarantined individuals (Q), diagnosed individuals who are being treated and isolated (D)	[46]
	Ia, Is, Qa, Qs, Ru, Ra, Rs	Undetected asymptomatic infectious (Ia), undetected symptomatic infectious (Is), detected and quarantined asymptomatic (Qa), detected and quarantined symptomatic (Qs), undetected recovered asymptomatic (Ru), recovered detected asymptomatic (Ra), recovered detected symptomatic (Rs)	[37]
	W1, R1, D1, W2, R2, D2	Hospitalized that never require an intensive care bed (W1), recovered from non-ICU (R1), deaths from non-ICU (D1), hospitalized that require an intensive care bed (W2), recovered from ICU (R2), deaths from ICU (D2)	[41]
	H1, H2	Confirmed cases who are quarantined at home (H1), confirmed cases who are hospitalized (H2)	[40]
	Ip, Ic	Primarily infected (Ip), chronically infected (Ic)c	[47]
Vaccination, testing or contact tracing	AC	Contact traced asymptomatic (AC)	[48]
	Eu, Ed	Undetected exposed (Eu), detected exposed (Ed)	[38]
	Su, Sv	Unvaccinated susceptible (Su), vaccinated susceptible (Sv)	[42]

^aUnsusceptible: a susceptible person can become unsusceptible due to factors such as the use of facemasks, hand washing, and SD (social distance)

^bSusceptible persons removed from isolation (S_T): isolated susceptible individuals, after a period, are released from isolation and transferred to compartment S_T

^cPrimarily infected: individuals that remain infectious within the reported duration of the infectious period after the incubation period; chronically infected: individuals that are less infectious but remain infectious and may be diagnosed for a longer duration