Safety and effectiveness of the booster dose of mRNA COVID-19 vaccines in people with multiple sclerosis: A monocentric experience

Background: Long-term data on the effectiveness and safety of the booster dose of anti-SARS-CoV-2 vaccines in people affected by multiple sclerosis (pwMS) are lacking, hence a retrospective monocentric study exploring these issues was undertaken. Materials and methods: PwMS who had received the booster dose of anti-COVID19 mRNA vaccines (either Comirnaty or Spikevax) according to the national regulation were included. The occurrence of adverse events or disease reactivation and SARS-CoV-2 infection were recorded up to last follow-up. Factors predictive of COVID-19 were explored using logistic regression analyses. A two-tailed p-value <0.05 was considered significant. Results: One hundred and fourteen pwMS were included: 80 females (70%); median age at the booster dose 42 years (range 21 - 73); 106/114 patients (93%) were receiving a disease-modifying treatment at vaccination. The median follow-up after the booster dose was 6 (range 2 - 7) months. Adverse events were experienced in 58% of the patients, being mild to moderate in most cases; 4 reactivations of MS were observed, two of which occurring within 4 weeks after the booster. SARS-CoV-2 infection was reported in 24/114 (21%) cases, occurring a median of 74 days (5-162) after the booster dose and requiring hospitalisation in 2 patients. Six cases received direct antiviral drugs. Age at vaccination and time between the primary vaccination cycle and the booster dose were independently and inversely associated with the risk of COVID-19 (HR 0.95 and 0.98, respectively). Conclusions: The administration of the booster dose in pwMS showed an overall good safety profile and protected 79% of the patients from SARS-CoV-2 infection. The observed association between the risk of infection after the booster dose and both younger age at vaccination and shorter interval period to the booster dose suggest that unobserved confounders, possibly including behavioural and social factors, play a relevant role in determining the individual propensity to get infected with COVID-19.