The virus doesn't spread because of a natural effect, but because infected people travel and take the virus with them. The speed of the spread is as fast as the

The virus doesn't spread because of a natural effect, but because infected people travel and take the virus with them. The speed of the spread is as fast as the fastest method of travel. If we have FTL travel, it travels faster than light. If we have airplanes, it travels as fast as them. So even if you don't watch land borders, shutting down airplane traffic to a country is highly useful: the virus cases are isolated to the border region of your country, and you can focus your attention to that part. Usually there is already infrastructure to close down land borders for cars and similar. So why not use that infrastructure? Besides stomping out the few cases inside its own borders or preventing travel from affected regions, it's the most useful measure a country can take. After that there are only inside country prevention methods, like curfews, forbidding car usage, etc.