Catching COVID-19 may raise the risk of developing autoimmune disease by 43% in the months following the infection, according to the largest study of its kind.
"The impact of this study is huge — it's the strongest evidence so far answering this question of COVID-19 and autoimmune disease risk," said Anuradhaa Subramanian, a research fellow in health informatics at the University of Birmingham, who was not involved in the study. The new research, which has yet to be peer reviewed, was posted Jan. 26 in the preprint database medRxiv.
Scientists previously linked COVID-19 to an increased risk of autoimmune disease, in which the immune system mistakenly attacks healthy parts of the body. However, this research was limited to small studies that focused on just a few conditions, such as autoimmune hemolytic anemia, which affects red blood cells, and Guillain-Barre syndrome, which affects nerve cells.
Now, researchers have analyzed the health records of 640,000 people in Germany who caught COVID-19 in 2020 and 1.5 million people who didn't knowingly catch the coronavirus that year to explore how the infection might affect the risk of developing any of 30 autoimmune conditions.
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They examined the rate at which people were newly diagnosed with autoimmune diseases in the three to 15 months after they tested positive for COVID-19. They compared these rates to those of the people who hadn't caught COVID-19. Roughly 10% of the participants in each group had preexisting autoimmune diseases.
Among the people with no history of autoimmunity, more than 15% of people who'd caught COVID-19 developed an autoimmune disease for the first time during the follow-up period, compared with roughly 11% of the people who hadn't caught COVID-19. In other words, the COVID-19 group had a 43% higher likelihood of autoimmune disease than the control group.
Among those with existing autoimmunity, those who caught COVID-19 had a 23% higher chance of developing an additional autoimmune disease in the follow-up period.
COVID-19 was most strongly linked to an increased risk of vasculitis, which causes inflammation of the blood vessels; the previously infected group had a 63% higher rate of a type of vasculitis called arteritis temporalis than the uninfected group did. Autoimmune-driven problems with the thyroid, a butterfly-shaped organ in the throat that releases hormones, and the skin condition psoriasis were also strongly linked to prior COVID-19 infection, as was rheumatoid arthritis, which causes swelling in the joints.
"These findings just cannot be ignored," Subramanian said. "We need to pursue research into how COVID-19 is potentially triggering autoimmunity because many people are continuing to suffer from the effects of COVID-19." There are several hypotheses as to how COVID-19 might trigger autoimmunity, and it's possible that different mechanisms affect different organ systems, the researchers noted.
"Understanding how COVID-19 impacts autoimmune disease risk will help in executing the prevention measures and early treatments to prevent associated morbidity and mortality," said Jagadeesh Bayry, a professor of biological sciences and engineering at the Indian Institute of Technology Palakkad who was not involved in the study.
Other viral infections, including influenza, have been linked to autoimmune disease, so more research is needed to establish what effects are specific to COVID-19, Bayry said. Future studies should also examine these links in diverse populations, beyond people living in Germany, Subramanian said.
Although the large sample size makes this a strong study, it is worth noting that it "only shows an association between COVID-19 and autoimmune disease but doesn't prove causality," said Dr. Atsushi Sakuraba, an associate professor of gastroenterology at the University of Chicago who was not involved in the research.
Another limitation is that there may have been people in the study's uninfected group who actually caught COVID-19 but developed little to no symptoms, and thus didn't know they'd been infected. The study also can't show whether different coronavirus variants are linked to a higher or lower risk of autoimmune disease, or how COVID-19 vaccination affects that risk.
