  In this study, we develop and analyze a nested multi-scale model for COVID
-19 disease that integrates within-host scale and between-host scale
sub-models. First, the well-posedness of the multi-scale model is discussed,
followed by the stability analysis of the equilibrium points. The disease-free
equilibrium point is shown to be globally asymptotically stable for $R_0 < 1$.
When $R_0$ exceeds unity, a unique infected equilibrium exists, and the system
is found to undergo a forward (trans-critical) bifurcation at $R_0=1$. Two
parameter heat plots are also done to find the parameter combinations for which
the equilibrium points are stable. The parameters $\beta, \pi$ and $\Lambda$
are found to be most sensitive to $R_0$. The influence of within-host sub-model
parameter on the between-host sub-model variables is numerically illustrated.
The spread of infection in a community is shown to be influenced by within-host
level sub-model parameters, such as the production of viral particles by
infected cells $(\alpha)$, the clearance rate of infected cells by the immune
system $(x)$, and the clearance rate of viral particles by the immune system
$(y)$. The comparative effectiveness of the three health interventions
(antiviral drugs, immunomodulators, and generalized social distancing) for
COVID-19 infection was examined using the effective reproductive number $R_E$
as an indicator of the effectiveness of the interventions. The results suggest
that a combined strategy of antiviral drugs, immunomodulators and generalized
social distancing would be the best strategy to implement to contain the spread
of infection in the community.
