It would have slowed the initial spread, but as we have seen, it only takes a single "superspreader" event to infect an entire country. Out of the various actions taken to fight the pandemic, closing borders was among the least effective. Basically, it only worked on islands and in combination with strong local actions (testing, tracing, quarantine, lockdowns, ...).
20 years ago wasn't the middle ages, air travel was a thing (9/11 was almost 20 years ago). In fact, it wasn't that different than it is today.
It would have slowed the initial spread, but as we have seen, it only takes a single "superspreader" event to infect an entire country. Out of the various actions taken to fight the pandemic, closing borders was among the least effective. Basically, it only worked on islands and in combination with strong local actions (testing, tracing, quarantine, lockdowns, ...).
20 years ago wasn't the middle ages, air travel was a thing (9/11 was almost 20 years ago). In fact, it wasn't that different than it is today.