Persistent “brain fog” — such as problems focusing or paying attention — is among the symptoms associated with Long COVID.

Persistent “brain fog” — such as problems focusing or paying attention — is among the symptoms associated with Long COVID.

What you need to know

“Brain fog” is a range of neurocognitive symptoms that can include forgetfulness and problems focusing, concentrating, and paying attention. Many people who have had COVID-19 have reported these and other persistent symptoms months after their initial illness — an extended condition also known as Long COVID.

In a new study supported by the National Institute of Allergy and Infectious Diseases, the National Heart, Lung, and Blood Institute, and the National Cancer Institute, researchers found that SARS-CoV-2, the virus that causes COVID-19, may live in a person’s gut long after infection. Changes in the gut microbiome have previously been linked to changes in cognitive function. In cases of brain fog, production of the chemical serotonin — which carries messages to nerve connections in the brain and helps with memory storage, food digestion, and sexual desire — is depleted.

What did the researchers do?

Researchers analyzed 1,540 people with symptoms of Long COVID. They gathered blood samples from 58 of the people who reported brain fog between 3 and 22 months after their initial SARS-CoV-2 infection and compared the results to results from 60 people who were in the early stages of an active infection and 30 people who reported symptom-free recovery from COVID-19.

The researchers found that those who had recovered from SARS-CoV-2 infection but experienced brain fog had much lower levels of serotonin than the other people — and that the drop in serotonin occurred right at the start of their infection.

While viral infection normally limits the body’s production of hormones and other key chemicals, levels usually bounce back to normal afterward. Researchers found that to be the case with every chemical they tested in this study — except for serotonin. They concluded that COVID-19 seems to be uniquely attacking serotonin production, which can lead to brain fog.

Why is this research important?

These findings could help to untangle some of the mechanisms behind Long COVID. Crucially, the findings may also provide vital biomarkers that can help doctors better test for — and potentially treat — the condition.

Where can I go to learn more?

The Relationship Between Chronic Viral Infection and Long COVID

  • Someone who experiences chronic viral infections may be more likely to develop Long COVID, although their specific symptoms may depend on the type of infection they have.

Researchers Identify Four Long COVID Categories

  • Using a computer algorithm to analyze the diagnoses of more than 20,000 patients who had COVID-19, researchers found that Long COVID symptoms tend to fit one of four categories or types.

Inflammation Pattern in the Brain May Cause Many Long COVID Symptoms

  • In addition to causing long-lasting organ damage, SARS-CoV-2 can set off a pattern of brain inflammation that may be linked to Long COVID symptoms.


Wong, A. C., Devason, A. S., Umana, I. C., Cox, T. O., Dohnalová, L., Litichevskiy, L., Perla, J., Lundgren, P., Etwebi, Z., Izzo, L. T., Kim, J., Tetlak, M., Descamps, H. C., Park, S. L., Wisser, S., McKnight, A. D., Pardy, R. D., Kim, J., … Levy, M. (2023). Serotonin reduction in post-acute sequelae of viral infection. Cell, 186(22), 4851–4867.e20.


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Page last updated: December 22, 2023