If you live in the U.S. travel restrictions have been put in place as a precaution to prevent covid infections. In European countries as of January 12, 2022, can travel to most European countries if fully vaccinated with specific exceptions being in covid hot zones such as U.K. and Switzerland have increased testing requirements for entry. For the Netherlands, Denmark and Ireland new Covid measures have been put in place to help handle the increasingly worse covid situation so travelers can expect more scrutiny. The U.S. requires a test with negative results one day before travel as well as unvaccinated foreign nationals from entering the country but unvaccinated Americans can return home while committing to a second Covid test 3 to five days after returning. Countries such as Switzerland, Croatia, and Austria require booster shots and 2nd shots within a certain time period in order to gain entry.

For more information on specific countries please look at the article this is sourced from.

source: https://www.nytimes.com/article/coronavirus-travel-restrictions.html