The Eastern Block wasn't that good at all at sealing borders. Many millions of people have emigrated to the West from Eastern Europe during the Cold War.
Millions I have a problem in believing, certainly once the iron curtain proper was put in place.
You don't need to resort to believing--either way. Just look at some statistics. You have all the web at your disposal, I suppose.
Wikipaedia says 13.3 million crossed east to west europe between 1950 and 1990, 75% of this movement was sanctioned so that makes for 4 million irregular crossings or about 100,000 per year. Currently the US is estimated to have 11.5 million illegal immigrants with about 0.5 million entering each year
Thanks for actually doing the work I was only bitching about!