COVID-19: Alzheimer's risk factor? Yes. Alzheimer's cause? Maybe. As research continues, the link between COVID, inflammation, and neurodegeneration becomes clearer.
Memory loss, confusion, cognitive impairment, and brain inflammation. Those aren’t just the signs and symptoms of Alzheimer’s disease and dementia. A year after a COVID-19 infection, people are 42 percent more likely to experience neurological symptoms. Scientists are researching the worrying links between the impact of COVID on brain health and Alzheimer’s disease. There are many studies emerging two years into the pandemic linking COVID infection to increased risk of Alzheimer’s, dementia or other neurological conditions.
But could this also validate the hypothesis that a bacterial or viral infection can lead to the development of Alzheimer’s? Let’s take a look at the research behind some of these ideas.
Alzheimer’s disease and COVID-19 inflammation
The immune system is an important contributor to brain health. Roughly half of the cells in the brain aren’t neurons, but instead astrocytes and microglia which regulate immunity within the brain.
When these cells aren’t working properly, it could lead to cell death and brain damage that is characteristic of cognitive impairment, Alzheimer’s disease, and dementia. One example: The brain’s microglia activate in response to beta-amyloid, releasing more signals which causes more inflammation and more damage.
These observations have even led to a new school of research into the disease — suggesting it should be treated like other immune disorders. There are substantial links between the immune dysfunction present across Alzheimer’s disease and what researchers have seen so far from COVID-19.
Late last year, scientists discovered that Alzheimer’s and severe COVID-19 infection shared a genetic risk factor called OAS1. People who have lower levels of OAS1 activity are more likely to have hyperactive immune cells running amok, whether it’s in the brain or the lungs. Another study compared the blood of people with COVID-19 to cognitive healthy individuals and those with Alzheimer’s. People with neurological symptoms during their infection had similar biomarkers of brain injury to people with Alzheimer’s disease — including higher levels of beta-amyloid and tau proteins as well as neurofilament light chain.
But COVID-19 and Alzheimer’s disease have far more than just inflammation in common.
COVID-19 infection mimics Alzheimer’s brain changes
Some researchers believe that viral and bacterial infections are the underlying cause of Alzheimer’s, dementia, and other neurodegenerative brain disorders. Earlier this year, scientists found compelling evidence that the virus behind mononucleosis — the Epstein-Barr virus — may be the cause behind multiple sclerosis. Studies of COVID-19 adds compelling evidence that viral infection could also cause Alzheimer’s and dementia.
A study published earlier this year in Nature Communications found that two proteins produced by the SARS-CoV-2 virus aggregate and clump — similar to beta-amyloid — causing damage to cells in the brain. The authors suggested that this may trigger the neurological symptoms such as memory loss, which also occur across Alzheimer’s disease. Furthering this idea, some patients who were hospitalized with COVID-19 showed increased rates of cognitive decline and neurodegeneration.
Even people who experienced an asymptomatic infection showed signs of cognitive impairment when compared to a cohort of people who were never uninfected.Another study from 2022 looked at the persistent cognitive symptoms developed by long COVID patients. These symptoms which persisted for months were found to be equivalent to 20 years of cognitive aging.
While a lot of this research is compelling, it does not directly prove that COVID-19 causes Alzheimer’s or dementia. It does however provide credence to the idea that COVID has an overwhelmingly negative impact on brain health and cognition.
COVID-19 infection increases the risk of Alzheimer’s or dementia
Now, more than two years into the pandemic, scientists can test whether the impact infection has on the brain persists and whether it increases the risk of developing other conditions like Alzheimer’s or dementia.
The largest study to date, published in The Lancet Psychiatry, tracked the post-COVID risk over the course of two years in 1.2 million people. The results were alarming, showing that, for a certain subset of people who recovered from COVID-19 two years ago, the chances of developing dementia rose from a 10-percent chance to a 13-percent chance — a one-third increase — along with statistically significant higher risk for other cognitive issues, from seizures to psychotic disorders.
Another study found no difference in the risk of developing Alzheimer’s when compared to other individuals recovering from other respiratory infections after one year. They also found that COVID-19 and other respiratory infections tripled the risk of receiving an Alzheimer’s diagnosis within the following year. While the results seem at odds, the latter study did not assess the risk of developing Alzheimer’s or any other dementia after two years. It is possible that COVID-19 has more of a lasting impact on brain health than other respiratory infections.
Dementia risk factor? Yes. Cause? Maybe.
COVID-19 leads to inflammation and changes in the brain that are similar to Alzheimer’s and other neurodegenerative disorders. While there is compelling evidence that COVID and other infections increase the risk of developing Alzheimer’s, it isn’t clear whether the infection itself is the cause.
It also isn’t clear whether it leads to the development of the disease in otherwise healthy people or if it speeds up an existing disease process.