Some countries don't allow unvaccinated travelers to enter, period. For example, travel to the majority of European countries is possible only to those who are vaccinated. If you're not vaccinated, make sure the country you want to visit will allow you entry.
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Here is a list of possible vaccines that you may need to get for the first time or boosters before you travel.
In much of Europe, the only travel document a US or Canadian citizen needs is a passport. (The US Passport Card works only for those driving or cruising to Canada, Mexico, Bermuda, and the Caribbean.)
Travellers who wish to enter certain countries or territories must be vaccinated against yellow fever ten days before crossing the border, and be able to present a vaccination record/certificate at the border checks.