The study's hypothesis was (paraphrasing): "The reason why people get the flu in the winter and don't get the flu in the summer is because of Vitamin D levels."
Since people are deficient in vitamin D in the winter (no sunlight), this leads to weakened immune systems (especially in the lungs), which opens people up for attack from the influenza virus and other respiratory infections (i.e. COVID-19).
Then this past week a breakthrough study was published that solidified our stance on Vitamin D: Calcifediol treatment and COVID-19-related outcomes.
Per the study's interpretation: In patients hospitalized with COVID-19, calcifediol (i.e. Vitamin D) treatment at the time of hospitalization significantly reduced ICU admission and mortality.
With COVID-19 still wreaking havoc on the world this seems like the perfect time to dive deeper.