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Several studies have raised concerns that vaccination might “induce a small increased risk of multiple sclerosis” and other acquired demyelinating syndromes (ADS). Now a new study suggests that vaccination may trigger symptomatic CNS ADS in people with existing subclinical demyelination.

Researchers compared 780 cases of CNS ADS and 3,885 controls. No associations emerged between CNS ADS and vaccination generally, or against hepatitis B or human papillomavirus specifically, during the three years following immunisation. But vaccination of any type more than doubled (odds ratio 2.3) the risk that CNS ADS symptoms would emerge within the following 30 days, but only among people less than 50 years of age.

“The short-term increase in risk suggests that vaccines may accelerate the transition from subclinical to overt autoimmunity in patients with existing disease,” the authors comment. In other words, vaccines may “hasten symptom onset” but “do not change the long-term risk” of MS or the clinically isolated syndrome (symptoms such as transitory optic neuritis or transverse myelitis).