At a billion doses per year it’s still 8 years to manufacture the vaccine for everyone. That’s assuming no supply chain issues. Moderna needs 2 doses which means only 500M protected per year which pages it to 16 years. I think the Oxford vaccine needs only 1 doses but because it uses a weakened virus there’s a chance a person develops covid-19.

Then you need to administer something like 300 doses per second globally, if you have 1B doses.

This is also assuming the virus doesn’t mutate during that time (it mutates very slowly but it could still happen).

The vaccine really needs to give multi-year protection otherwise it won’t be viable in the long run.