Season of infectious mononucleosis as a risk factor for multiple sclerosis: A UK primary care case-control study

Background. Infectious mononucleosis (IM) and vitamin D deficiency are both risk factors for multiple sclerosis (MS). Objective: We wished to establish if IM in the winter months when vitamin D levels are low may be a greater risk factor for MS than IM in the summer months. Methods: We identified all patients with MS diagnosed aged 16-60 in a large primary care database in the United Kingdom and matched each by age, sex, general practice and observation period with up to six controls. We identified a coded diagnosis of IM prior to the index date (date of diagnosis). Logistic regression was used to calculate the odds ratio for prior IM exposure in cases versus controls and for winter versus summer exposure in cases and controls with prior IM exposure. Results: Based on 9247 cases and 55,033 matched controls (246 and 846 with prior IM respectively), IM was associated with the development of MS (OR 1.77, 95%CI 1.53-2.05) but there was no evidence that IM in the winter as opposed to summer was associated with developing MS (OR 1.09, 95%CI 0.72-1.66). Conclusion: We found no evidence that the season of IM influences the risk of subsequent MS.