It doesn't matter if they are a closed society, which they are NOT. They are closed ONLY to the Western world and western proxies in Asia, Japan, S. Korea, but they allow the Chinese come and go anytime.

Having said that, unless their country is protected by a giant antiseptic sterilized dome, there's no actual way of preventing a microscopic organism from entering a place, specially a highly aggressive weaponized enhanced virus that can survive outside an organism for days, maybe even weeks.

Nature ALWAYS finds a way to deliver something to your door. Different species of carriers, bugs, birds, fish, water, air.