What is the current situation as of October 2021?

Austria

Only fully-vaccinated travelers from the U.S. can enter Austria and without quarantine requirements. 

Those who are not vaccinated will have to pre-register their trip, present a negative COVID-19 test on arrival and quarantine for 10 days. 

Belgium, the Netherlands

Only fully-vaccinated travelers from the U.S. can enter Belgium and the Netherlands. Testing and quarantine is required on arrival.

Czech Republic

Fully-vaccinated travelers from the U.S. can enter the Czech Republic. Keep in mind, there are no direct flights between the U.S. and the Czech Republic so visitors would need to check travel measures for the countries you are transiting through.