I'm currently working in the NHS, have struggled the last two winters, and am finding increasing numbers of people around me are taking vitamin D.
It is difficult to get in your diet, although I regularly eat mushrooms and oily fish, both of which are contain some. As temperate creatures living in a climate which is unsuitable for us, we all need to move further South.
I cracked just before Christmas and bought some. We're both taking a tablet every day. Vitamin D tablets can cause some problems and need to be taken with food. So far it seems okay, I've perked up a bit, but its hard to separate the vitamin D out from everything else.