EU Digital Covid Certificate: Everything you need to know
https://edition.cnn.com/travel/article/eu-covid-certificate-travel-explainer/index.html
I wonder why Japan has not yet allowed visitors or returning citizens to enter Japan without any restriction. That's because Japan don't know what kind of vaccination certificate around the world. Since Japan's many agencies are infamous for bureaucracy, they can't accept documents which are genuine but have trivial difference from designated one.
For example, an immigrant with the negative certificate as jpeg image was rejected to enter Japan, because pdf is designated file format.
