Even before, likely everyone was deficient in Vitamin D during winter. You "charge up" in the summer, but then you start dropping below the beneficial levels lo

Even before, likely everyone was deficient in Vitamin D during winter. You "charge up" in the summer, but then you start dropping below the beneficial levels long before the sunny spring starts again. It's part of the reason people get sicker in the winter, feel more "blue", and so on.