Are adult immunizations absolutely mandatory? Or can one opt out of them?

Choices/consequences. If they are not required as part of your employment & you are not under court order, you can do as you wish. You can roller skate on the freeway too.As long as there is international air travel you are subject to exposure to infections from anywhere in the world. We have hundreds die yearly from flu that did not get a shot . Tetanus is rare but a particularly painful way to die.
No. According to CDC yes.

Please read all complications & preservatives used to make your decision.

It depends on the diseases also.

Some vaccines are mandatory.
Adult . Adult immunizations are not mandatory; however a company may require an employee to be vaccinated depending on their guidelines for employment i would suggest before deciding if you will be vaccinated or not, do your research, ask questions, and make sure you make an informed decision with the help of your medical care provider.