Here's why COVID gives some people 'brain fog' – and why variants make a difference
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Scientists are a step closer to understanding how COVID can cause symptoms such as 'brain fog' and how the virus first enters the brain.
COVID-19 causes respiratory symptoms, but can also affect the brain, with many people experiencing brain fog or cognitive deficits, and some developing neurological conditions after infection.
New lab-based research by the Francis Crick Institute in London has shown that different variants of the virus have very different effects on brain cells.
Alize Proust, senior laboratory research scientist in the Tuberculosis Laboratory at the Crick, and first author, said: "There's been lots of speculation about how COVID-19 caused neurological symptoms, and whether it happens indirectly via inflammation, or if the virus can directly attack brain cells or cross the barrier.
"We've shown that it can do both of these in the lab, but the effects are variant-specific. This has implications for what we've already seen.
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"People can develop neurological disorders after a COVID-19 infection, sometimes months after. As we age, our ability to generate new neurons diminishes, so if the virus kills some brain cells, this could speed up the start of a disease affecting the brain.
"Our work could be important for clinicians to understand what type of damage a variant is causing, and to look for specific treatments."
Researchers aren't completely sure how the virus enters the brain, and the main theories are that it enters through the nerves that transmit smell or through the blood-brain barrier, a selective barrier that separates the brain and the bloodstream.
Brain cells and a 3D model of a blood-brain barrier were exposed to different strains of SARS-CoV-2: wild-type (the original variant from Wuhan), alpha, beta, delta, eta and omicron.
The brain cells investigated were pericytes, astrocytes, endothelial cells and microglia – cells that support nerve cells and control how permissible the blood-brain barrier is to allowing molecules and cells to cross.
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The scientists showed that all variants caused stress to brain cells, stopping them from working as well.
The wild-type virus killed all cell types except astrocytes, whereas alpha and beta only killed pericytes, and omicron killed endothelial cells and pericytes.
The researchers also looked at how well the variants crossed the model blood-brain barrier.
The wild-type virus and, to a lesser extent, omicron were able to disrupt the integrity of the barrier while the other variants could not.
The researchers suggested that the virus damaging the barrier and increasing its permeability could lead to immune cells entering the brain and inducing inflammation.
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