Certain countries require proof of vaccination on arrival, and whether it applies often depends on which countries a traveller passed through. The reasoning concerns local ecology rather than the individual's own protection.
The requirement protects the destination
A country free of a particular disease but home to the insect that transmits it is described as receptive: the disease is absent, yet the conditions for it to spread are present.
An arriving traveller carrying an infection could allow transmission to begin locally, after which the disease would no longer depend on further arrivals to persist.
The certificate requirement is therefore a border control measure aimed at the country's own population, which is why it is enforced by immigration rather than by health services.
Itinerary matters more than nationality
Because the concern is where a traveller has been, requirements are usually written in terms of countries visited or transited within a defined period before arrival.
A passenger who spent hours in an airport in an affected region can therefore fall within the rule while one flying direct from elsewhere does not.
This is why the same passport can face different requirements on two trips to the same destination, depending entirely on the routing chosen.
The certificate is an internationally agreed document
Proof is provided through a standardised international certificate whose format is agreed between states so that a document issued in one country is recognised in another.
Validity periods are defined within that framework, and a certificate generally becomes effective only after a waiting period following administration rather than immediately.
That delay is the reason arrangements have to be made well before departure rather than in the final days before a flight.
Airlines enforce the rule before the border does
Carriers are held responsible for passengers arriving without valid documentation, so checks frequently happen at the departure gate rather than on arrival.
Being turned away at that point is common where a connection through an affected region was booked without the requirement being noticed.
Where a certificate cannot be held for a medical reason, formal exemption documentation exists, and acceptance of it is decided by the destination rather than by the traveller.
Requirements change and vary by country
Lists of affected regions are revised as outbreaks appear and recede, so guidance more than a few months old may no longer reflect the current position.
Rules also differ sharply between neighbouring states, and a regional generalisation is a poor basis for planning a multi-country trip.
Anything concerning a traveller's own health, including suitability and timing, belongs with a qualified health professional, and the entry requirement should be confirmed with the destination's own authority.