It's not the vitamin D, although this was widely believed a long while ago, there are much more productive papers that show that melatonin is much closer linked.
Looking at the progression of covid waves in the northern hemisphere during winter, also showed a general progression from north to south as the winter progressed, vitamin D levels was a red herring as people who get more sun, generally have higher vitamin D levels, but it's the raised melatonin that is significant.