After recent major snowstorms, frigid temperatures continue to grip much of the United States — and that cold, dry, and rainy weather can significantly boost flu transmission. More than 15 million flu cases have already been reported early in the 2026 season.

The flu virus survives longer in low humidity and cold conditions, while rapid weather shifts and increased indoor gatherings further drive infection rates. Studies show that flu activity often peaks about one month after the coldest, driest, or most humid and rainy periods.

New research suggests that people with low levels of vitamin D may be more likely to develop severe flu infections. The study, published in The American Journal of Clinical Nutrition, analyzed health data from 36,258 participants in the UK Biobank, a large long-term research database.

Researchers found that individuals with severe vitamin D deficiency — defined as blood levels of 15 nmol/L — had a striking 33% higher risk of being hospitalized for a respiratory tract infection compared with those whose vitamin D levels were 75 nmol/L or higher. For every 10 nmol/L increase in vitamin D levels, the risk of hospitalization dropped by 4%.

The Food and Nutrition Board at the National Academies of Sciences, Engineering, and Medicine states that vitamin D levels of 50 nmol/L (20 ng/mL) or higher are sufficient for most people, while the risk of deficiency increases at levels below 30 nmol/L (12 ng/mL). To convert nmol/L to ng/mL, divide by 2.5.

Infectious disease experts note that vitamin D plays a role in supporting both innate immunity — the body’s first line of defense — and adaptive immunity, which helps the immune system learn to fight specific pathogens. However, they also emphasize that the study focused on people with severely low vitamin D levels and found an association — not proof of cause — between deficiency and serious infection.

The National Institutes of Health (NIH) recommends a daily vitamin D intake of 600 IU (15 mcg) for people ages 1 to 70, including pregnant and breastfeeding women, and 800 IU (20 mcg) for adults over age 70. These amounts are designed to maintain healthy blood levels and support overall immune function. For most people, this intake is sufficient to achieve blood levels above 50 nmol/L (20 ng/mL), which is considered adequate for bone and general health.

Symptoms of vitamin D deficiency can be subtle and easily overlooked. Common signs include fatigue, frequent infections, muscle weakness, bone or back pain, and mood changes such as depression. In more severe cases, deficiency may contribute to poor wound healing, hair loss, bone loss, or an increased risk of fractures. Anyone concerned about deficiency should consult a healthcare provider for proper testing and guidance.

Vitamin D levels can be increased through sun exposure and diet. Foods rich in vitamin D include cod liver oil, trout, salmon, mushrooms, and fortified milk and cereals. Experts recommend having blood levels tested before starting supplements, since vitamin D is fat-soluble and can accumulate in the body. Excessive intake may raise calcium levels in the blood, leading to health problems.

Dr. Thomas Russo, chief of infectious diseases at the University at Buffalo School of Medicine in New York, says the best flu-prevention strategy still starts with proven measures.

“The ideal flu-prevention program is getting your flu shot — yes, even now, because flu season doesn’t end until April or May — and wearing a mask in crowded indoor settings,” Russo says.

“A vitamin D supplement is one piece that may help,” he adds. “There’s little harm as long as you don’t overdo it.”