The coronavirus may "impair" a patient's brain function in severe cases, a study has suggested.
Early research implies the infection is mild in four out of five incidences, however, it can trigger a life-threatening disease called COVID-19.
Initially considered a respiratory virus, the infection is now known to affect many parts of the body, including the brain.
To learn more, scientists from John Hopkins University in Baltimore analysed the brain tissue of 15 people who died with a confirmed coronavirus infection.
Five (33%) of the deceased had large bone marrow cells in their brain's capillaries, a phenomenon not seen "prior to this pandemic".
Although unclear, the coronavirus may alter signalling in the body, causing bone marrow cells to enter the bloodstream, reaching the brain.
The cells may then block blood flow through the vital organ's vessels, "potentially resulting in an atypical form of neurologic impairment".
Watch: Can coronavirus impact the brain?
A loss of taste or smell is recognised as a common symptom of mild coronavirus, highlighting how the infection can affect the brain.
In more serious cases, a patient may endure confusion and even a loss of consciousness.
Once a former coronavirus patient tests negative for the infection, some develop brain fog and fatigue as a symptom of long COVID, further demonstrating the vital organ's link to the pathogen.
Previous studies have analysed the brain tissue of people who died with the coronavirus after experiencing "neurologic abnormalities".
These have not consistently shown inflammation or "marked changes" that are usually associated with a viral infection in the brain, the John Hopkins scientists wrote in the journal JAMA Neurology.
Read more: Blood-thinning drugs cut COVID death risk
The presence of the coronavirus' genetic material in the vital organ has also been "minimal or absent".
Believing brain tissue analyses "might provide some insight", the John Hopkins scientists studied samples from 15 coronavirus victims.
These samples were compared against those of two individuals who died without the infection, but were of a similar age to the coronavirus victims and showed signs of brain injury caused by oxygen deprivation.
Five of the coronavirus victims showed signs of megakaryocytes – large bone marrow cells that give rise to blood-clotting platelets – in their brain, which was later confirmed in the laboratory.
The two cadavers without the coronavirus had oxygen deprivation-related brain changes, but no megakaryocytes.
"Prior to this pandemic, the study neuropathologists had not seen megakaryocytes in brain vessels and we find no reference to this in the literature," wrote the John Hopkins scientists.
A recent report similarly found the same cells in the brain of another coronavirus victim, with the vital organ's tissue having died due to an inadequate blood supply.
This suggests megakaryocytes may have been circulating in the victim's brain, entering the "parenchyma" following a bleed, wrote the John Hopkins scientists. The parenchyma is generally defined as the functional cells of an organ, rather than the supportive or connective tissue.
"Multiple lines of evidence" suggest dysfunction to the cells that line blood vessels may contribute to severe coronavirus complications, according to the scientists.
Megakaryocytes have also been spotted in coronavirus patients' lungs and other organs.
Altered cell signalling may recruit megakaryocytes from the bone marrow into the bloodstream, "somehow permitting them to pass through the lungs".
The scientists only analysed a "minute portion" of the cadavers' brains via randomly-collected samples, suggesting "the total burden could be considerable".
The team concluded megakaryocytes may block capillaries, reducing blood flow "in a distinct pattern, potentially resulting in an atypical form of neurologic impairment".
Watch: Can you catch coronavirus twice?
How coronavirus may affect the brain
The coronavirus' affect on the brain was flagged relatively early in the pandemic.
In July 2020, scientists from University of College London (UCL) reported a "higher than expected number of people" with the infection showed signs of brain damage, some of whom did not have severe respiratory symptoms.
One expert then warned it "remains to be seen" whether the coronavirus causes the same brain damage as Spanish flu.
The 1918 Spanish flu pandemic left some patients enduring viral encephalitis, brain swelling that can occur if an infection enters the central nervous system.
Some survivors went on to develop viral Parkinsonism, defined as symptoms similar to Parkinson's disease, such as shaking and stiff movements.
Earlier UCL research also found a "higher than expected number of [coronavirus] patients with stroke", which was put down to the infection making blood "stickier".
More recently, scientists from the US National Institute of Neurological Disorders and Stroke (NINDS) analysed the brain tissue of 19 people who died with the coronavirus.
The results revealed signs of inflammation and bleeding in the brain regions that control a person's breathing, heart rate and sense of smell.
On closer inspection, the NINDS scientists found these regions were made up of thinned, clotted and "leaky" blood vessels, which are usually associated with strokes.
Experts have previously said "time will tell" how the coronavirus affects the brain, with some complications being "very likely", but rare.