[Mediterraneo10]

The WHO yellow fever certificate is not digital, it is just a piece of paper. Plus, many of the countries which ostensibly require it don't check it carefully or at all (and in West Africa, it is not unusual for the soldier checking it to be illiterate and unable to actually grok the details on it). So, this old-school vaccine proof doesn't pose the risk of being used for ad targeting that worries the GP.

[Pyramus]

Yes, in rare cases that might happen but in general that sounds like a trope. In fact, I've heard stories of people being denied entry and also getting vaccinated on arrival in a back room at the airport, which is as dodgy as it sounds.

Do you speak from experience?

[Mediterraneo10]

Yes, I speak from repeat personal experience in both Africa and South America. That checking of the certificate in South America has dwindled is well known. Sure, some people may have bad luck, but there is a reason that many holidaymakers are no longer even aware that there is a rule on the books.

The certificate is commonly checked in Africa, but as I said, often the official on the border checking it is not capable of understanding the details – they just look for the paper with the familiar color and logo. Also, it has been common for travelers unable to get the yellow fever vaccine in their home country (historically supplies in Eastern Europe have been scarce, for instance) to simply forge the certificate, which is easily done. The WHO is aware that some amount of certificates will be forgeries, but nevertheless believes that the policy of requiring vaccination will be enough to reduce the risk of outbreaks.