It will be a lot easier for some countries to implement than others. Australia is a good example. Qantas has stated their intention to require vaccination for international travel. That's easy for them - they're starting from zero, so a short-term "trickle" of passengers is better than none. If United/American/Delta/Air Canada are left to their own devices to impose a deadline it may never happen, since it will cut into their current business. If a majority of people are not yet vaccinated, that's a majority of their potential customers. Even government intervention won't guarantee anything. The Canadian federal government has imposed a January 7 date for having a mandatory negative test for arriving international passengers. The airlines have refused thus far to participate at the origination point on the grounds they don't have jurisdiction, aren't health experts or law enforcement officials. As a result the testing will be done here on arrival and the airlines may not be forced to transport those not complying back. I suspect that issue will be decided by a court long after it becomes a problem.For international flights just like visa requirements are different for different countries, health certificate requirements will be different for different countries, and just like for travel permits are checked before boarding, health permits will also be checked by airlines before boarding. They do it because they are on the hook at the arrival end for anyone that arrives without proper documentation, to haul them back to where they came from free of charge.