People infected with COVID-19 show significant changes to regions of the brain that affect memory and smell along with increased cognitive decline, new research has found.
In one of the biggest-yet COVID-19 brain imaging studies, researchers looked at scans from 785 people aged 51 to 81 in the UK - before and after mostly mild COVID-19 infection.
The study was carried out by researchers at the University of Oxford, University College London (UCL), Imperial College and the National Institutes of Health.
The peer-reviewed research found significant changes to the areas of the brain that affect memory, smell and a reduction in brain size in people who caught the virus.
Participants received two brain scans on average 38 months apart. They also underwent cognitive tests.
Of the 785 patients, 401 participants tested positive for COVID-19 between their two scans, and 15 were hospitalised.
The researchers compared the scans to those of the remaining 384 patients and identified various long-term effects following infection, including a greater reduction in grey matter thickness in regions of the brain associated with smell and memory of events.
Additionally, people who caught the virus displayed evidence of tissue damage in regions associated with smell, and an average reduction in whole brain sizes.
Participants who tested positive on average also showed greater cognitive decline between their two scans, associated with the atrophy of a brain region that is linked to cognition.
The authors say the documented effects were still seen after excluding the 15 people who had been hospitalised with COVID-19, implying even mild illness may have consequences for the brain.
The authors also performed a control analysis on people with pneumonia to show the changes were specific to COVID-19, and not due to the generic effects of catching a respiratory illness.
Auckland University Professor Maurice Curtis said the study shows there are significant long term consequences from catching COVID.
"The smell pathway and the memory pathway in the brain are connected and these are the same pathways affected in some dementias including Alzheimer's disease.
"From the available data the researchers also conclude that COVID-19 causes the shrinkage whereas being infected with the seasonal flu did not cause brain shrinkage.
"This study demonstrates that there is a long-term consequence to getting COVID-19 and it highlights the importance of taking all measures possible to reduce COVID-19's impact on the body and especially the brain."
University of Otago Neurodegenerative and Lysosomal Diseases Laboratory research fellow Dr Indranil Basak said the results are worrying.
"This is a well-performed longitudinal study, the results look very interesting, but worrying as well. It is a very good example of what COVID-19 can do to the human system long term."
Dr Basak said the research shows the virus has an effect on the brain which could explain Long COVID.
"There are definitely several cases where patients infected with the virus started getting neurological symptoms like dizziness, disturbed consciousness, headache, loss of smell and taste, seizure, encephalitis and there have been links with Parkinson's disease.
"From this research and our own, it is clear there is an effect on the brain from COVID-19 infection, and this could lead to some Long COVID effects. We still don't know how to treat this, because no one has looked at it yet. But we do know that the virus directly, or indirectly, can affect the human brain."
New Zealand is currently in the midst of an Omicron outbreak with the seven-day rolling average of cases sitting at 17,921.
