The first effect will be the complete closing of borders of China to all other nations. Will some people slip through? Of course, on foot and in private jets and planes and on boats. Of course this is bound to happen.
But, I think the most likely thing that will happen to China is a Great Depression economically. However, the Chinese government also has tools that most other governments don't at their disposal in these kinds of situations.
So, it is possible that the world's nations will have a much worse time of it than China will long term regarding all this.
First of all, China has 1.4 Billion people, so even if they alone lost 300 million people they likely could still be okay economically long term. However, other nations on earth are far less prepared on all levels to deal with all this and many 3rd world nations could be completely wiped out on many different levels if this virus hits them very hard.
China would still have over 1 billion people to work and to serve which might even operate better than their government and nation do now because they would have less overpopulation than now.
The nations most likely to survive all this the best are the best organized and the most wealthy. For example, the European Union, and the United States likely will emerge from all this in the best shape, and likely the United Kingdom also has the kind of governmental system and health care system where it too can survive something like this.
However, with Trump as president I'm not sure what is going to happen when we basically have a king defeating all our democratic principles in dealing with all of this.
In other words I greatly question whether Trump can cope with all this and save the American people properly in this kind of situation.
So, this situation likely might be the end of his presidency if the truth actually is allowed to come out.
Population of China: current, historical, and projected population, growth rate, immigration, median age, total fertility rate (TFR), population density, urbanization