Passive immunization. This works in general. Before antibiotics became widely available, treatment with '
convalescent serum' was a treatment that worked for some infections. Infusing antibodies (gammaglobulin, IgG, immunoglobulin G) is still used to block some infections e.g. Hepatitis B in newborns and Chicken pox/Herpes Zoster (VZV). This has already been proposed for COVID-19 and of course will be something the ultra rich and powerful are already lining up.
There is really no way, short of a vaccine, to 'boost' one's immune system. At least not anyway that has been scientifically proven AFAIK. Vitamins are pretty much useless if you eat a decent diet and most studies show either no benefit or harm.