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Overall, people with multiple sclerosis (MS) were not at increased risk of relapse after receiving booster doses of the COVID-19 vaccine, according to a large nationwide study in France.

Among over 100,000 patients with MS, there was no increased risk of relapse after one, two, or three booster doses of COVID vaccination (combined incidence rate ratio [IRR] 0.97, 95% CI 0.91-1.03, P=0.30), reported Xavier Moisset, MD, PhD, of the Université Clermont Auvergne in Clermont-Ferrand, France, and colleagues.

This was observed across various subgroups, including MS patients younger than 50, those with a duration of MS less than 10 years, and those who were using disease-modifying treatments, they detailed in .

Notably, there was an increase in the risk for relapse after a booster dose in one subgroup: patients who had two or more relapses in the previous 2 years (IRR 1.28, 95% CI 1.07-1.53, P=0.006).

A small increase in the relapse risk cannot be excluded after a booster dose (IRR 1.39, 95% CI 1.08-1.80) for patients with high MS activity, especially when not treated, the authors noted.

"Data for the mRNA vaccines were acquired for very large populations and are robust," Moisset and team wrote. "These vaccines can be used without any worry about the risk of relapse to provide booster doses to patients for whom they are warranted."

However, they did urge caution for patients with the highest inflammatory activity in the previous 2 years, noting that they should first receive disease-modifying treatments.

"During the COVID-19 pandemic, patients with MS were shown to have a risk of severe infection," co-author Emmanuelle Leray, PhD, also of the Université Clermont Auvergne, told 番茄社区app. "Due to this excess risk, it is highly important that patients with MS get vaccinated."

While several previous case reports and case series suggested a just after COVID vaccination, Leray noted that her study team has not observed this in clinical practice, but "such results may worry patients and prompt them not to follow the recommendations."

"Our findings are reassuring and have direct implications for the management of patients with MS in the context of pandemics," she added.

A 2023 French study showed that several vaccines were not tied to MS flares, reinforcing 2019 guidelines from the American Academy of Neurology, which recommended annual flu shots and other immunizations for those with MS.

"Based on the study findings, clinicians should feel confident recommending standard vaccines with favorable safety profile in treated MS patients, emphasizing their protective effects against vaccine-preventable infections and infection-related relapses," wrote Hesham Abboud, MD, PhD, and Mauricio Farez, MD, MPH, both of Case Western Reserve University in Cleveland, in an . "Avoiding booster doses and nonmandatory vaccines in untreated highly active patients should be considered."

Clinicians could also recommend vaccines after initiating disease-modifying treatments for highly active patients, "especially agents that do not dampen the vaccine immune response such as natalizumab [Tysabri]," they added.

Abboud and Farez emphasized that more information is needed on relapse severity, and any connection between vaccines and non-relapse MS activity, like MRI changes and disease progression. Another gap in the study, they wrote, was the overall risk of post-vaccinal isolated demyelinating attacks in those without MS, or those who may go on to develop it later in life.

For this study, Moisset and colleagues used the French National Health Data system, linked to the National COVID-19 Vaccination Database, to identify 102,524 patients with MS who received at least one dose of a COVID vaccine in 2021. Mean age was 54.3, and 71.8% were women. Mean duration of MS was 13.5 years.

A self-controlled case series method was used to evaluate the risk of relapse associated with COVID vaccines in the 45 days after vaccination. The associated risk was evaluated after one, two, or three booster doses. MS relapses were identified by a validated algorithm based on hospital admissions and the use of high-dose corticosteroids.

The authors acknowledged that there were limitations to their study, including the exclusion of benign relapses and the inclusion of pseudo-relapses. They also may have counted hospitalizations of patients with MS incorrectly as a relapse.

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