Depends on what country. I lived in Angola (West Africa) for two years and every time you entered the country you had to provide your yellow vaccine card to prove you had your vaccines for diseases like yellow fever. If you did not, they would pull you aside and give you the shot right then and there. Although countries are not requiring the COVID vaccine right now, they will once it is widely available. Right now, not everyone can get it so it is not required but testing is. For example, my husband was planning a trip in May to go sailing in the BVIs with some friends, until they found out that they would be required to test negative before they went. Then they would be tested when they arrived in the BVIs and would only be allowed to sail to four islands until their test results came back, which they said would take four days. That would allow them only 2 days to sail where they wanted to go. Trip has been cancelled. When vaccinations are widely available, this scenario will change and you will need to provide your proof of vaccination or vaccination passport.