This study will be a single center, retrospective, pilot study to determine the effect of Rebif (interferon beta-1a) and Tecfidera (dimethyl fumarate) on infections on total lymphocyte counts, grade of lymphopenia, Cluster of Differentiation 4 (CD4) and Cluster of Differentiation 8 (CD8) counts and ratios in subjects aged 50 years and above with Relapsing-Remitting Multiple Sclerosis (RRMS).

Inclusion Criteria:

• RRMS subjects who have been treated with interferon beta 1a SC 44 micro-gram thrice a week and dimethyl fumarate in inpatient or outpatient settings during the period from January 1, 2015 to December 31, 2015.
• Subjects with at least one year of laboratory values post-index date to analyze lymphocyte counts, and CD4 and CD8 counts. The date of first blood draw for laboratory tests after January 1, 2015 will be defined as the subject's index date.

Exclusion Criteria:

- Subjects who have been on treatment for less than 1 year after their index date.

The study will include subjects aged 50 years or older with relapsing RRMS, identified from a feasibility assessment conducted at a single center in United States, who were treated with either interferon beta-1a SC 44 micro-gram or dimethyl fumarate between January 1, 2015 to December 31, 2015.