A new study finds that people with multiple sclerosis may not have a higher risk of relapse after COVID-19 vaccination. The study’s authors said the findings are reassuring that these vaccines can be used without the worry of risk of relapse. The absence of such a risk is encouraging for people with MS so they may receive booster shots when needed.

The study by researchers at Clermont Auvergne University in Clermont-Ferrand, France, included 124,545 people with MS in France. They had been living with MS for an average of 14 years and were followed for 45 days after vaccination, as potential vaccine-induced relapses generally occur within 28 days after vaccination.

During the study, 102,524 people, or 82 percent, received at least one dose of a COVID-19 vaccine. A total of 95 percent received a second dose and 59 percent received an additional booster dose. Participants received one or more of the following vaccines: Pfizer BioNTech, Moderna, AstraZeneca or Janssen.

In the 45 days following vaccination, researchers looked at relapses requiring treatment with high-dose corticosteroids. After adjusting for other factors that could affect the likelihood of a relapse, such as time of year and the effect of disease-modifying therapy, researchers found that COVID-19 vaccination did not increase the risk of severe relapse. These results remained consistent after each dose.

To confirm the findings, researchers compared people who had relapses to those without. Similarly, they found no increased risk of vaccine exposure. They identified a small decrease in relapse risk after vaccination.

Researchers said particular caution is needed for patients with the highest inflammatory activity, who should first receive disease-modifying treatment before their booster vaccination. People who were untreated and those with a highly active disease showed a small increased risk after the third vaccine dose. The risk was highest if both factors were combined.

A limitation of the study is that researchers only looked at relapses requiring corticosteroids, so relapses that were not reviewed by neurologists or not needing the use of corticosteroid therapy were not considered.

The study is published in the journal Neurology.