Rwat, S. I., and N. A. A. Ahmad. “Backward Bifurcation and Hysteresis in a Mathematical Model of COVID19 With Imperfect Vaccine”. MATEMATIKA: Malaysian Journal of Industrial and Applied Mathematics, vol. 39, no. 1, Apr. 2023, pp. 87-99, https://matematika.utm.my/index.php/matematika/article/view/1458.