All you can do is try to stay reasonably fit and keep your weight under control. Being overweight is a major risk factor for covid which is partly why there have been so many deaths in the USA where a lot of people are obese. Smoking is another risk factor of course.
Nobody has any immunity to Delta/Omicron unless they have antibodies to these viruses and the only way to get antibodies is to either catch the virus or be vaccinated, as both these will cause your body to produce antibodies. Unfortunately Omicron is escaping immunity which means even if you're vaccinated and/or have contracted it, you can still catch it again, but you won't be as sick.
Eat healthy, try to exercise and follow the health guidelines about masks, sanitiser and physical distancing is about the best we can do! Plus ring for medical assistance if you have serious symptoms like difficulty breathing.
No harm in trying vitamins if you want to..
