Table 1.

Description and initial values of model variables

Variables	Description	Initial conditions	Source
$N_H$	Total population size of humans	29, 825, 964	(Worldometers 2020)
$S_H$	Number of susceptible humans to both COVID-19 and CHIKV	$N_H$	Assumed
$E_1$	Number of humans exposed to COVID-19	$\left[0,,,N_H,\times ,{10}^{-6}\right]$	(Tsay et al. 2020)
$A_1$	Number of asymptomatic COVID-19 individuals	0	Assumed
$I_1$	Number of COVID-19- infected individuals	0	Assumed
$R_1$	Number of recovered from COVID-19 individuals	0	Assumed
$P_1$	Number of perished with COVID-19 individuals	0	Assumed
$I_{R2}$	Number of infected to CHIKV after recovery from COVID-19	0	Assumed
$E_2$	Number of humans exposed to CHIKV	0	Assumed
$A_2$	Number of asymptomatic CHIKV individuals	0	Assumed
$I_2$	Number of CHIKV infected individuals	0	Assumed
$R_2$	Number of recovered from CHIKV individuals	0	Assumed
$P_2$	Number of perished with CHIKV individuals	0	Assumed
$I_{1R}$	Number of infected to COVID-19 after recovery from CHIKV	0	Assumed
$I_{12}$	Number of dually infected with both COVID-19 and CHIKV	0	Assumed
$R_{12}$	Number of recovered from both COVID-19 and CHIKV	0	Assumed
$P_{12}$	Number of perished by both COVID-19 and CHIKV	0	Assumed
$N_m$	The total population of mosquitoes	$\left[{10}^3,,,5,\times ,{10}^4\right]$	(Sanchez et al. 2018)
$X$	Number of susceptible mosquitoes to CHIKV	$\left[{10}^3,,,5,\times ,{10}^4\right]$	(Sanchez et al. 2018)
$Y$	Number of mosquitoes exposed to CHIKV	0	(Sanchez et al. 2018)
$Z$	Number of CHIKV-infected mosquitoes	1	(Sanchez et al. 2018)