This is particularly something that did not take me by surprise when I heard about.
When the Covid-19 happened, we had to stay on lockdown almost in every country. We could just observe the safety precautions. Lucky enough, vaccines were produced after a long wait. These seem to be the best option to protect yourself and others from the infection as such taking the shot of the vaccine became a requirement to travel from one country to another.
A vaccine card is made available to each person that takes the vaccine. This card is checked each time you are asked of your vaccination status.
