The Association Between <scp>MOG</scp> ‐ <scp>IgG</scp> Positivity and <scp>mRNA SARS</scp> ‐ <scp>CoV</scp> ‐2 Vaccination in Denmark: A Self‐Controlled Case Series Study

PURPOSE: Spontaneous reporting of a possible link between myelin oligodendrocyte glycoprotein (MOG) antibody-associated disease (MOGAD) and mRNA SARS-CoV-2 vaccination warrants further examination. Hence, the objective of this study was to investigate the association between MOG-IgG positivity as a proxy for MOGAD and mRNA SARS-CoV-2 vaccination in the Danish population. METHODS: We applied the self-controlled case series study using Danish nationwide health registers. The study population consisted of all Danish residents who were offered an mRNA COVID-19 vaccination and had a first positive MOG-IgG test result between November 1, 2020 to December 31, 2023. RESULTS: Among 5.9 million individuals eligible for SARS-CoV-2 vaccination, we identified 111 incident events of MOG-IgG positivity in the study period of which 28 (25%) occurred in children. We found no association between mRNA SARS-CoV-2 vaccination and MOG-IgG positivity in adults, but an increased risk-although based on small numbers-was found in children below 18 years of age (incidence rate ratio = 8.07, 95% confidence intervals: 3.07-21.21). If the observed association reflects a causal effect, the estimated number of excess MOG-IgG positive events attributable to mRNA COVID-19 vaccination in Denmark was 4.38 per 1 000 000 exposed children. CONCLUSIONS: These results suggest an increased risk of MOG-IgG positivity after mRNA SARS-CoV-2 vaccination in children, but not in adults. The number of additional MOGAD events attributable to mRNA SARS-CoV-2 vaccination in children is very low and further studies are needed to confirm these findings.