how does sunlight immune system and covid 19 interact

For thousands of years, humans have recognized that the sun plays a role in the emergence and transmission of viruses
For thousands of years, humans have recognized that the seasons play a role in the emergence and transmission of certain illnesses, including viruses. “Annual epidemics of the common cold and influenza disease hit the human population like clockwork in the winter,” write the authors of a 2020 review paper from a team at the Yale University School of Medicine. They also point out that two deadly coronaviruses — first SARS, and now Covid-19 — both emerged during the winter months. “[This indicates] that the winter environment promotes the spread of a variety of respiratory virus infections,” they write.
While the seasonality of many common respiratory illnesses is well-established, it may surprise some to learn that experts haven’t nailed down the exact causes of this phenomenon. For the most part, they tend to agree that a mix of environmental factors — such as temperature and humidity — play a role in pathogen transmission. So does human behavior; people are more likely to crowd together indoors during the winter months, and stagnant indoor air can increase the likelihood of spreading germs. But that Yale review also lists sunlight as a possible explanation for the seasonality of certain pathogens.