Haven't seen this clear info before, very nice pictures etc:

Vasculitis: Granulomatosis with Polyangiitis and Microscopic Polyangiitis

"Prevalence of GPA in New Zealand is similar to that in Norway, suggesting that incidence of GPA in the southern hemisphere may be reciprocal to its incidence in the northern hemisphere.3,4"

So is Weg more common in the hemispheres?