Let's say one country or a region could take action to contain the corona virus (eg: South Korea, Wuhan China). How could they remain protected while other countries or regions still have the virus? Do they have to stay isolated until no one in the world has the virus? Or if protection can only going to come through newly developed immunity by getting infected, does that mean every one in the world at some point has to get infected until a vaccine is invented?
does that mean every one in the world at some point has to get infected until a vaccine is invented?
No. There are many infectous diseases which are only regional, or have been almost eradicated. Countries free of (this or that) disease need to act quickly on imported cases, if the disease is highly infectuous.
On the other hand, if the spread of a pathogen in a (say closed) region is slowed down substantially, people who have developed immunity in this region will generally make the reproductive factor of the epidemic lower - and thus make it easier to contain its spread.
You may have a look at the last simulation in: https://www.washingtonpost.com/graphics/2020/world/corona-simulator/ where the epidemic can in many cases self-extinguish itself by that effect.