Then you run into studies like this:

Temporal Variation of Onset of Relapses in Multiple Sclerosis: Results from the Northern and Southern Hemispheres in the MSBase Registry

Orla Mary Gray, Damien Jolley, et. al.

OBJECTIVE: To determine if there is a temporal variation in onset of relapses using the MSBase registry, a large, multi-centre cohort study of MS outcomes. To compare the time of onset of relapses in the northern and southern hemispheres.

BACKGROUND: Previous studies into time of onset of relapses have suggested that relapses are seasonal, with more relapses in spring and fewest in winter. The proposed mechanism is that reduced vitamin D levels at spring onset precipitate relapses. However small numbers, differing diagnostic criteria and the involvement of single regions limited these studies.

DESIGN/METHODS: Data was extracted on 16th July 2008. The dataset comprised 7,860 cases with all forms of MS from 33 centres in 16 countries, including 25,784 documented relapses. Relapses with 1st January recorded as day of onset were excluded, leaving 22,684 in total including 5,542 first demyelinating events. Relapses were stratified by hemisphere of residence and compared by season, quartile and month of onset. Statistical analysis was performed using chi-squared test.

RESULTS: 22,684 relapses (19,775 northern, 2,909 southern) were included. Relapses were significantly more common in spring in the northern hemisphere (P<0.0001) and autumn in the southern hemisphere (P<0.0001). June had the highest number of relapses than any other month in either hemisphere (P<0.0001). These results were replicated with analysis of the 5,542 first demyelinating event in MS cases (4,801 northern, 741 southern).

CONCLUSIONS/RELEVANCE: A highly significant temporal variation in onset of relapses is present in both northern and southern hemispheres. However, peak relapse incidence does not occur in the same season in the northern and southern hemispheres.

That doesn't make sense . . . since June is "fall" in the southern hemisphere (following a ton of sun), and is after a few months of tolerable sunshine that we would most be exposed to in the northern hemisphere. Many of us might stay indoors in July/Aug, cause it's too hot, but June is a great month for having seen enough sun ....

You'd think it would be late fall or winter that we would have the most relapses, if it was due to lack of vitamin D.

Cherie