See the supplimentary material of this paper [1] They split the I compartment into two, one for the documented infected and one for undocumented infected.

[1] Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2) Li et al. DOI: 10.1126/science.abb3221 https://science.sciencemag.org/content/early/2020/03/24/scie...