Due to the pandemic, NPA notes, proper nutrition is becoming harder to access and more important: A vitamin D deficiency weakens the immune system, which is crucial during this time, and Americans ordered to stay home and practice social distancing are exercising less and eating less nutritious food. COVID-19 is also exacerbating existing problems, NPA says: More than 23 million Americans live in food deserts, making it hard for them to access nutritious food even in the best of times.