Here is a list of countries that are open to U.S. citizens. Many require visitors to complete a health form and present a recent negative Covid-19 test result. Many also demand proof of full vaccination, and some places, like Switzerland, Croatia and Austria, are requiring booster shots or second doses within a specified period of time. People who have recently visited or traveled through countries with high numbers of Covid-19 cases may be denied entry or be subject to additional restrictions.