Incidence of multiple sclerosis in multiple racial and ethnic groups

To determine whether the incidence of multiple sclerosis (MS) varies by race/ethnicity in a multiethnic, population-based cohort.

We conducted a retrospective cohort study of more than 9 million person-years of observation from the multiethnic, community-dwelling members of Kaiser Permanente Southern California health plan from January 1, 2008 to December 31, 2010. Incidence of MS and risk ratios comparing incidence rates between racial/ethnic groups were calculated using Poisson regression.

We identified 496 patients newly diagnosed with MS who met McDonald criteria. The average age at diagnosis was 41.6 years (range 8.6-78.3 years) and 70.2% were women. The female preponderance was more pronounced among black (79.3%) than white, Hispanic, and Asian individuals with MS (67.8%, 68.1%, and 69.2%, respectively; p = 0.03). The incidence of MS was higher in blacks (10.2, 95% confidence interval [CI] 8.4-12.4; p < 0.0001) and lower in Hispanics (2.9, 95% CI 2.4-3.5; p < 0.0001) and Asians (1.4, 95% CI 0.7-2.4; p < 0.0001) than whites (6.9, 95% CI 6.1-7.8). Black women had a higher risk of MS (risk ratio 1.59, 95% CI 1.27-1.99; p = 0.0005) whereas black men had a similar risk of MS (risk ratio 1.04, 95% CI = 0.67-1.57) compared with whites.

Our findings do not support the widely accepted assertion that blacks have a lower risk of MS than whites. A possible explanation for our findings is that people with darker skin tones have lower vitamin D levels and thereby an increased risk of MS, but this would not explain why Hispanics and Asians have a lower risk of MS than whites or why the higher risk of MS among blacks was found only among women.