And the odds of a vaccine being literally "mandatory" in democratic countries are essentially nil. No one is breaking down doors and holding families down to get their vaccines.
(What is possible is that schools and even workplaces require proof of vaccination once one is widely available if there is still a widespread virus risk. That could arguably be fairly coercive but it's important to distinguish from being "mandatory.")
While a widespread vaccine requirement to be physically present in places that will be important to many is a fairly big stick, it's still a far cry from vaccinating people by force. Certainly I might well favor the former but would be horrified by the latter.
I also think many people in high risk categories (age, preexisting health issues, essential or health care workers, teachers, other professions involving crowds and risk) will be very eager to vaccinate. It doesn't have to be mandatory for high demand and high use.
Where did the "mandatory" come from?
If a vaccine is promising but unproven, the sensible thing is to make it available to those who want to take the risk.
I agree that for a mandatory vaccine, you want a high level of certainty.