  As new COVID-19 variants continue to emerge globally, we develop an SISI-type
mathematical model to determine whether the transmission risks arising when
such a new variant enters Indonesia can be mitigated by solely accelerating the
country's ongoing four-dose vaccination programme. We begin by determining the
model's basic reproduction number, as well as the model's equilibria and their
stability. Subsequently, employing parameter values representing the country's
situation as of March 20, 2023, we conduct a numerical sensitivity analysis in
two simulated cases corresponding to two different levels of the new variant's
transmission. The results show that, a satisfactory mitigation relying solely
on vaccinations necessitates a drastic acceleration in the low-transmission
case, and proves unachievable in the high-transmission case. Accordingly, we
recommend that the acceleration of the ongoing four-dose vaccinations be
carried out in conjunction with other intervention measures, such as
improvements of the vaccine's efficacy and the disease's recovery rate.
