Dr. Anthony Fauci says most immune-boosting supplements do nothing, except for vitamin D and vitamin C. The 79-year-old director of the National Institute of Allergy and Infectious Diseases said in an interview, in clinical studies, most "so-called immune-boosting supplements didn't really help people unless they have some sort of deficiency," according to CNBC.

However, he said vitamin D deficiency has an "impact on your susceptibility to infection. So I would not mind recommending, and I do take it myself," he said.

The infectious disease expert added vitamin C is "a good antioxidant" and "if people want to take a gram or two at most of vitamin C, that would be fine."

Other experts agree. Researchers at the University of Chicago found an association between vitamin D deficiency and the likelihood of becoming infected with the coronavirus, according to UChicago News.

"Understanding whether vitamin D deficiency changes COVID-19 risk could be of great important locally, nationally and globally," said Dave Meltzer, lead author of the study. "Vitamin D is inexpensive, generally very safe to take, and can be widely scaled."

Experts say half of Americans are thought to be deficient in vitamin D, according to UChicago News, particularly Blacks and Hispanics.

According a study published by the National Institutes of Health, vitamin C is a potent antioxidant that contributes to immune defense to prevent respiratory and systemic infections, researchers said.

Fauci added the best way to boost your immune system is to get good sleep and exercise, according to CNBC. Fauci, a member of the White House COVID-19 task force, said while he often works 20-hour days, he tries to get rest and drink water on those long days.

He also said he makes time to power walk 3½ miles daily, adding he gave up his 7-mile lunch run because it was too hard on his body, according to CNBC.