I had a skeptical mom. I had to explain to her the science/facts/risks re: COVID19 before she stopped going out.
She understands asymptomatic carriers/spreaders, the growth curve, and risk of hospital overwhelming now. She also knows my father is a risk for serious complications (including death), given his heart condition. Both parents fully understand the risks and are not going out for anything non-essential.
I've had to really emphasize social distancing too.
Although we can never force a person to do something, I think a combination of fear and love can help. Tell them how much you love them and cannot stand the thought of something happening to them. And explain the dangers. I really had to work on this part. Again, I broke down all the science for her and as she read the news herself (reinforcing what I had said), she understood the concern and took it seriously.
There were times I angrily pleaded with my mom. I kept at this. She finally did come around.
I think a combination of love and fear in messaging can help drive home the point. Some people may respond more to one than the other. I just kept trying (constantly pleading with them). My mom seemed to respond most to my dad being vulnerable.