The single intervention which would most benefit the health of most humans is the oral supplementation of vitamin D3 (cholecalciferol). Vitamin D is necessary to the function of every cell of the immune system.
Low vitamin D levels are associated with severe COVID-19 and increased likelihood of hospitalization and a stay in the ICU.
For most people who weigh over 100 pounds, 5000 international units (IU) per day of vitamin D3 will achieve a blood level in the normal range of 30-100 ng/mL (nanograms per milliliter). However, people with more body fat, or who have had their gallbladder removed might need more. 10,000 IU per day might be required. Checking a blood level after 3 months of supplementation can guide daily dosing.
Vitamin D is a fat-soluble vitamin, which slowly builds up in the body and slowly declines. Levels rise with sun exposure in summer months and fall in the darker winter months. Highest blood levels are typically found in September, and lowest levels in March-April in the northern hemisphere.
Vitamin D is actually a hormone, essential for immune function, bone support, calcium metabolism, kidney health and cardiovascular function. Low levels are associated with diabetes, metabolic syndrome, obesity, coronary artery disease and stroke.
Ultraviolet light from the sun causes a photochemical reaction in skin that creates active vitamin D. The majority of people who do not work or play outdoors are deficient in vitamin D. Black skin pigmentation reduces vitamin D production, which might partly explain why blacks in the US fared worse from COVID.
Above and below the sub-tropics, the winter sun is too weak to produce vitamin D, even if you bare your skin at midday.
Many of us learned a lot more about vitamin D during the COVID pandemic, because it was one of the few readily available and safe interventions to reduce illness, reduce disease severity and improve survival using prophylactic, early and even late treatment. 114 treatment studies are compiled here, and 86% show benefit for COVID from vitamin D in various formulations and dosages. 
From early 2020 it was apparent that people with low vitamin-D levels had higher mortality rates from COVID, and it stood to reason, so I gave away Vitamin D3 (5000 IU) to coworkers and patients from early that year. (I gave away over 40,000 doses of vitamin D in 2020. This was well received, and people told me later that they were providing it to their families after that.) 
In the spring of 2020 a prospective, randomized study was performed in Spain using an immediately bioavailable form of vitamin-D, calcifediol, in patients hospitalized for COVID-19. Those supplemented did much better, informing my own practice. 
The question of how to get blood levels of vitamin D normalized quickly has been studied since the 1940s. A comprehensive review found that a single dose of 300,000 IU of vitamin D3 (cholecalciferol, the kind sold over the counter) resulted in normalization of vitamin D levels within a week, and maintained adequate levels for up to 3 months. 
In addition to vitamin D, the immune system is further supported by supplementation with vitamin K-2 (100-200 micrograms per day), magnesium and zinc at low doses, vitamin C, and vitamin B2 (riboflavin), which is purchased cheaply in B-complex vitamins. Further immune system support comes from a healthy intestinal bacterial “microbiome”, which typically comes from eating fresh vegetables, raw and cooked, and some fermented foods.
A very good one-hour presentation of the metabolic benefits of vitamin-D for the immune and other systems, related to COVID-19 but broadly applicable, has recently been posted at FLCCC by Paul Marik, MD. 
*The information presented here is educational and not medical advice, to help people adjust their lifestyles to better meet their needs.
 Ancient Vitamin-D Protection: https://www.johndayblog.com/2021/12/ancient-vitamin-d-protection.html
 Effect of calcifediol treatment and best available therapy versus best available therapy on intensive care unit admission and mortality among patients hospitalized for COVID-19: A pilot randomized clinical study
 Understanding Vitamin-D, Paul Marik MD and endocrinologist Sunil Wimalawansa MD, 1 hr presentation with slides, followed by Q&A https://covid19criticalcare.com/understanding-vitamin-d/