A severe case of the coronavirus may accelerate the onset of Alzheimer's disease, research suggests.
Although initially considered to be an airway infection, symptoms including fatigue, muted senses and headaches suggest the virus can also affect the brain.
At least one in 10 people who overcome the coronavirus go on to endure long COVID, with lingering complications despite the infection being cleared from the respiratory tract.
To better understand the coronavirus' affect on cognitive health, medics from the New York University Grossman School of Medicine analysed 310 patients who were admitted to hospital with severe complications.
All cognitively sharp before catching the infection, up to 158 of the patients developed so-called toxic-metabolic encephalopathy (TME), generally defined as brain dysfunction in the absence of a disease.
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Blood samples revealed these patients had higher levels of the tell-tale proteins that form plaques and tangles in an Alzheimer's-afflicted brain.
Speaking of the study, presented at the Alzheimer's Association International Conference, one expert has admitted the results "may seem concerning", but stressed the extent to which the coronavirus "might play into our risk of getting dementia" is unclear.
"These new data point to disturbing trends showing COVID-19 [the disease caused by the coronavirus] infections leading to lasting cognitive impairment and even Alzheimer's symptoms," said Dr Heather Snyder, from the Alzheimer's Association.
"With more than 190 million cases and nearly 4 million deaths worldwide, COVID-19 has devastated the entire world.
"It is imperative we continue to study what this virus is doing to our bodies and brains. The Alzheimer's Association and its partners are leading, but more research is needed."
Coronavirus aside, around 850,000 people are living with dementia – an umbrella term for a loss of brain function – in the UK. Of these patients, between 50% and 75% have Alzheimer's disease.
Of the 310 patients – all of whom had a confirmed coronavirus infection – 158 endured neurological symptoms, the most common being confusion due to TME.
Levels of the proteins tau and amyloid beta were significantly higher in the TME patients, compared to the 152 patients without neurological symptoms.
An abnormal build-up of tau causes tangles to form within brain cells, while amyloid deposits settle around these cells.
These protein accumulations affect how chemical messages are sent between brain cells. Over time, the vital organ shrinks, which usually starts with the area responsible for memories.
The TME patients also had higher levels of markers that are linked to inflammation and brain injury.
It is unclear if the coronavirus itself enters the brain or the infection triggers an inflammatory immune response that damages the vital organ.
"These findings suggest patients who had COVID-19 may have an acceleration of Alzheimer's-related symptoms and pathology, however, more longitudinal research is needed to study how these biomarkers impact cognition in individuals who had COVID-19 in the long term," said study author Dr Thomas Wisniewski.
Speaking of the study, Dr Richard Oakley – from the Alzheimer's Society – said: "Although on first glance these results about the link between COVID-19 and problems with cognition may seem concerning, at this stage we don't know how suffering from COVID symptoms such as brain fog or loss of taste or smell might play into our risk of getting dementia.
"Research looking into the long-term impact of coronavirus is absolutely vital, especially if there's a potential association with the diseases causing dementia."
Also presented at the Alzheimer's conference, medics from the University of Texas in San Antonio reported that people who endure a persistent loss of smell after overcoming the coronavirus are more likely to experience forgetfulness or other cognitive issues.
A University of Thessaly, Greece, study has also revealed experiencing cognitive decline after the coronavirus is associated with poor physical fitness.
Writing in the journal JAMA Network Open, medics from Oslo University Hospital reported that even a mild case of the coronavirus more than quadruples a person's risk of enduring memory problems eight months later.
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