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Whether you were hospitalized with COVID-19 or were only briefly slowed down by a mild case, the virus may have changed your brain tissue, according to a new study.

Researchers led by a team from the University of Oxford compared brain scans of people who had contracted COVID-19 with those who had not and found marked differences, according to the study, which was published Monday in the journal Nature.

Of the 785 British participants, ages 51 to 81, a little more than half caught the virus with most cases coming when the alpha variant was prevalent. The images came from the UK Biobank database, which dates back to 2014, thus providing ideal pre-pandemic “before” scans.

“Using the UK Biobank resource, we were in a unique position to look at changes that took place in the brain following mild – as opposed to more moderate or severe – SARS-CoV-2 infection,” said the study’s lead author, Gwenaëlle Douaud, a professor in the department of clinical neurosciences at the University of Oxford. “Despite the infection being mild for 96 per cent of our participants, we saw a greater loss of gray matter volume, and greater tissue damage in the infected participants, on average 4.5 months after infection.”

The researchers compared subjects with matching characteristics when it came to age, sex, baseline test date and health issues when evaluating scans.

The majority of changes in the brain were found in areas tied to the sense of smell, according to Douaud. All of the negative effects were pronounced in older people.

The study found that people who had COVID-19 also showed a general reduction in brain volume and performed worse during cognitive follow-up tests.

Data suggested that the tissue damage found in the 15 people who were hospitalized with COVID was worse than in those people who experienced mild cases, but Douaud said the sample size was too small to be conclusive.

What causes the brain changes is not yet known, the researchers said, but some of the working theories include COVID-related inflammation of the tissue and a type of “sensory deprivation” that may be caused by the virus’ effect on the sense of smell.

“A key question for future brain imaging studies is to see if this brain tissue damage resolves over the longer term,” Douaud said.