  A coronavirus disease 2019 (COVID-19) model with quarantine and standard
incidence rate is first developed, then a novel analysis approach for finding
the ultimate lower bound of COVID-19 infectious individuals is proposed, which
means that the COVID-19 pandemic is uniformly persistent if the control
reproduction number $\mathcal{R}_{c}>1$. This approach can be applied to other
related biomathematical models, and some existing works can be improved by
using it. In addition, the COVID-19-free equilibrium $V^0$ is locally
asymptotically stable (LAS) if $\mathcal{R}_{c}<1$ and linearly stable if
$\mathcal{R}_{c}=1$, respectively; while $V^0$ is unstable if
$\mathcal{R}_{c}>1$.
