Sadly, you usually can’t save people from themselves. As with all things which are beyond your control, just tell the truth and hope for the best. “Covid is deadly and the vaccine can protect you. I don’t want you to get sick or die.” Once you’ve done that, the rest is out of your hands. The same is true when you ask someone to wear a seatbelt or try to quit smoking. You can do only what you can do. But try not to lose yourself in anger (although that frustration is understandable.)