Immune response to the third COVID-19 vaccine dose is related to lymphocyte count in multiple sclerosis patients treated with fingolimod

BACKGROUND: The majority of multiple sclerosis [MS] patients treated with fingolimod fail to develop a protective level of IgG humoral and adaptive cellular immune responses following full BNT162b2 SARS-CoV-2 vaccination. OBJECTIVE: To compare the efficacy of the third COVID-19 vaccine dose in vaccine non-responders fingolimod-treated MS patients. STUDY DESIGN: This is a prospective 3-month, single-center, randomized clinical trial. METHODS: Twenty relapsing MS patients who had been on fingolimod therapy ≥ 12 months and failed to develop humoral IgG immune response to 2-dose Pfizer BNT162b2 COVID-19 vaccination were randomized into two groups: fingolimod-continuation group and fingolimod-discontinuation group. Humoral and memory cellular immune responses were assessed within 1 and 3 months following the third Pfizer BNT162b2 vaccine dose and compared between the groups. RESULTS: A higher rate of patients in the fingolimod-discontinuation group [n = 8/10] compared to fingolimod-continuation group [n = 2/10] developed positive SARS-COV-2 IgG. Median IgG titer 1 month following the third dose was 202.3 BAU/ml vs. 26.4 BAU/ml, respectively, p = 0.022. The development of IgG humoral response correlated with absolute lymphocyte count. Specific SARS-COV-2 memory B cell and T cell immune responses were not detected in both groups, either at 1 month or 3 months following the third COVID-19 vaccine dose. CONCLUSIONS: Short period of fingolimod treatment discontinuation was associated with the development of humoral protection but not with adaptive cellular immunity. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00415-022-11030-0.