As others have noted, the question cannot be answered simply on "what countries or airports permit sterile transit" (ie. international transit). There are many factors that apply, particularly the nationality(ies) of the specific passenger and which passport(s) the passenger is using as well as the visa status of the passenger and the countries being traveled to/from, length of time of the transit interval, other stops in the itinerary, and others.
For example, see the guidance provided at this link:
Airport Transit Visa (germany.info) for an example of how complicated it can be.
If you can provide more specifics about the passenger(s) and the itinerary then some folks here might be able to give more information.