Scientists are becoming increasingly concerned with the emergence of a syndrome termed “long COVID,” where a significant percentage of sufferers of COVID-19 experience long-lasting symptoms.
The risk of long COVID is no longer thought to be directly linked with either age or the initial severity of the COVID illness. So younger people, and people with initially mild COVID, can still develop long-COVID symptoms.
Some long-COVID symptoms begin quickly and persist, whereas others appear well after the initial infection has passed.
Symptoms include extreme fatigue and ongoing breathing complications.
What particularly concerns us as neuroscientists is that many long COVID sufferers report difficulties with attention and planning — known as “brain fog.”
So how does COVID affect the brain? Here’s what we know so far.
How does the virus get to our brains?
There’s evidence connecting respiratory viruses, including influenza, with brain dysfunction. In records of the 1918 Spanish flu pandemic, reports of dementia, cognitive decline, and difficulties with movement and sleep abound.
Evidence from the SARS outbreak in 2002 and the MERS outbreak in 2012 suggest these infections caused roughly 15-20 percent of recovered people to experience depression, anxiety, memory difficulties, and fatigue.
But there’s data suggesting it may “hitchhike” into the brain by way of nerves that connect our noses to our brains.
Researchers suspect this because, in many infected adults, the virus’s genetic material was found in the part of the nose that initiates the process of smell — coinciding with the loss of smell experienced by people with COVID.
How does COVID damage the brain?
These nasal sensory cells connect to an area of the brain known as the “limbic system,” which is involved in emotion, learning, and memory.
In a UK-based study released as a pre-print online in June, researchers compared brain images taken of people before and after exposure to COVID. They showed parts of the limbic system had decreased in size compared to people not infected. This could signal a future vulnerability to brain diseases and may play a role in the emergence of long-COVID symptoms.
The virus also activates the immune system, and in some people, this triggers the production of toxic molecules, which can reduce brain function.
Although research on this is still emerging, the effects of COVID on nerves that control gut function should also be considered. This may impact digestion and the health and composition of gut bacteria, which are known to influence the function of the brain.
The virus could also compromise the function of the pituitary gland. The pituitary gland, often known as the “master gland,” regulates hormone production. This includes cortisol, which governs our stress response. When cortisol is deficient, this may contribute to long-term fatigue.
This was a recognized phenomenon in patients diagnosed with SARS, and in a disturbing parallel with COVID, people’s symptoms continued for up to one year after infection.
There are major unanswered questions about long COVID that require investigating, including how the disease takes hold, what the risk factors might be and the range of outcomes, and the best way to treat it.
We must begin to understand what causes the wide variation in symptoms. This could be many factors, including the viral strain, severity of the infection, the effect of pre-existing disease, age, and vaccination status, or even the physical and psychological supports provided from the start of the disease.
While there are many questions about long COVID, there’s certainty about one thing: we need to continue doing everything we can to prevent escalating COVID cases, including getting vaccinated as soon as you’re eligible.