Losing the mandate of heaven, I suspect.
Communist societies appear strong but are not resilient. The end may very well be near for the CCP.
I agree that this is definitely part, even a major part, of their calculus. But losing the Mandate of Heaven would have to occur because of something specific - and a massive pandemic that the “infallible” CCP was unable to stop would be such a thing. But even if losing power to an internal revolt was their first concern, keeping it while a devastating disease (whether natural or engineered, it doesn’t matter) killed off a large number of their people and shut down trade would also weaken them a lot...and they are not interested in being weakened in any way.
Either way, they are extremely interested in containing a very bad disease - and that fact is not a cause for optimism.