I hope my personal testimony helps you (and others). Years ago, I get tired easily, feeling depressed too. A blood test showed low count of white blood cells. My nutritionist at the time, a very up-to-date professional, requested a specific blood test for Vitamin D. Which was really low because I never sunbathe in sufficient quantity. She prescribed three 7,000UI pills a week and I never felt tired again. After 3 months she asked me to keep getting the suplemment "for life" (or getting more sunbath time). You can take Vitamin K2 to help absorb better the Vitamin D.

So I researched on the Internet about "Terrain Theory", which argues that we don't need a vaccine, but to keep our immune system always up to date. Meaning, a diet with anti-inflammatory foods (turmeric, ginger, red pepper, low carbohydrate to mention some, research for anti-inflammatory food) and vitamin D supplement. BTW, the diseases that kill most are caused by inflammation caused by excess of carbohydrate.

Also, if you have black skin, Vitamin D becomes more imperative because black skin cannot absorb enough Vitamin D from the Sun.