Closing borders might not stop the spread, but it certainly slows it down if there is a major population of sick people in one country, but in the other country

Closing borders might not stop the spread, but it certainly slows it down if there is a major population of sick people in one country, but in the other country the population of sick people is smaller. The numbers of detected and undetected cases are different, but definitely not uncorrelated. The less people have the disease, the more you can focus onto containment measures. If you detect one case in a country of 30 million, you can test everyone who had contact with that person and maybe even their entire street. However, if there are 10 000 cases in that same country, you suddenly have a problem. Even if you assume constant spreading rates with an exponential model of patient number increase, where after a period P, the number of cases doubles, then with 10 case, after P you have 20 cases. If you have 100 cases because you left your border open and, say 50 000 people from the country with many cases got in, you'll have 200 cases after period P. Closing borders is an efficient method to slow down the spread. Of course you need to accompany these cases by within-country measures like China is taking, but you can isolate them to affected regions. What's important though is that you prevent travel in any fashion as much as possible, because the virus doesn't travel by itself, it needs humans to travel for it.