Whether going to or through, a traveler's country of destination may have implemented its own health screening requirements.
To stay informed in a constantly evolving pandemic situation, it is highly recommended that you refer travelers to official government or foreign institution websites of the country(ies) they are crossing so that they may have access to accurate information in real time.
Inside Europe
Staying within the EU will make travelling a great deal easier because all European countries accept paper or digital health certification formats, attesting either
1. individual's vaccination certificate
2. and/or negative PCR test
3. or recovery status regarding COVID-19
provided they comply with the EU Digital COVID Certificate (EUDCC).
Nevertheless, each country is free to apply its own health screening requirements and strengthen their requirements towards citizens coming from sensitive regions.
Outside Europe
Beyond ongoing discussions between UE and UK or UE and US towards mutual recognition of countries’ certificates, no common regional or global standard health certificate framework or solution has been agreed yet. Nevertheless, airline companies may provide travelers with a solution (paper or digital) that comply with the health screening requirements of the country they are crossing. Also it would be wise for them to carefully read the documents that are provided along with their booking as they may contain important guidelines that will ultimately improve their overall health certification process.