Study Shows No Causal Relationship Between COVID-19 Vaccination and Myositis

According to a study published in Arthritis Care & Research that analyzed the Veteran’s Health Administration (VHA) records found there is no evidence to suggest COVID-19 vaccination increases the risk of developing idiopathic inflammatory myositis (IIM). 

The case-control study that focused on veterans diagnosed with IIM for the first time identified 89 patients who met strict criteria, including having at least two IIM-related Interstitial Classification of Diseases codes, a minimum of one year of VHA enrollment prior to their first IIM diagnosis and chart reviews confirming the incident nature of the condition. Each of the 445 cases were matched to five control patients who shared similar characteristics, including age, gender, clinic specialty visits and year of visit, but had no IIM diagnosis. The researchers examined the timing of COVID-19 vaccinations relative to the onset of IIM, looking specifically at whether the patients had received their first COVID-19 vaccine within 30 or 90 days before the index date (the date of IIM diagnosis or equivalent for controls).

Among the 89 IIM cases, seven individuals (7.9 percent) had received the vaccine within 30 days of diagnosis when compared to 29 controls (6.5 percent). When the timeframe was expanded to 90 days before the index date, 11 case patients (12.4 percent) and 68 controls (15.3 percent) had been vaccinated. Multiple other time-interval analyses also failed to reveal any significant link between COVID-19 vaccination and new-onset IIM.

According to the researchers, these results are important because they are the first to systematically compare the risk of developing myositis after COVID-19 vaccination to a carefully matched population. And, they said, the study adds to the growing body of evidence that COVID-19 vaccines are safe with respect to rare autoimmune muscle disorders.