We probably need an infection specialist to fill in the details but historically, these fast moving viruses tend to blow through the population quickly, hammer the most vulnerable, and fade away just as quickly. Some people have natural immunities or resistance to the new virus, and all those who were exposed but had mild symptoms are now resistant. This brings the epidemic to an end as it becomes self limiting.
This is behind a paywall but describes in detail how the SARS Cov 2003 pandemic was isolated and stopped. It is the older brother of Covid-19 with very similar origin and biology.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2636331/