Association of vitamin D levels with multiple sclerosis activity and progression in patients receiving interferon beta-1b

Fitzgerald, Kc; Munger, Kl; Köchert, K; Arnason, Bg; Comi, Giancarlo; Cook, S; Goodin, Ds; Filippi, Massimo; Hartung, Hp; Jeffery, Dr; O'Connor, P; Suarez, G; Sandbrink, R; Kappos, L; Pohl, C; Ascherio, A.