And one common symptom seems to be a preview of cognition troubles to come.

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What we do know: COVID-19 can manifest as anything from asymptomatic to debilitating to deadly, and it's something that 35.6 million Americans and counting (10.8% of the population) have tested positive for, according to the U.S. Centers for Disease Control and Prevention.

What we don't know: What the long-term health impacts will be on the 35 million (so far) who have survived their battle with the coronavirus.

So researchers are building databases of survivors to learn as much as they can—as quickly as possible—to try to mitigate as much future damage as possible.

Initially, we believed that most of the residual damage would exist within the lungs, since COVID-19 is a respiratory virus. But scientists are learning that the collateral damage might be more systemic.

New research presented at July's Alzheimer's Association International Conference 2021 reported connections between COVID-19 and persistent cognitive deficits, including increased speed of onset for Alzheimer's disease. Along with the respiratory and gastrointestinal symptoms of COVID-19, many individuals who test positive also suffer from short-term or long-term neuropsychiatric symptoms, such as loss of taste and smell and "brain fog."

None of the studies have had time to be peer-reviewed yet, which is the standard for trusted scientific research, so their findings are considered preliminary. That being said, they had some significant findings and suggest notable connections between the virus and the brain.

Initial findings from one study published in Alzheimer's & Dementia on survivors who reside in Greece and Argentina suggests that older adults who have recovered from COVID-19 often suffer persistent cognitive impairment, including long-term lack of smell. More than half of the 300 individuals who had tested positive in the past still struggled with forgetfulness for at least 3 to 6 months after recovering, and 1 in 4 had other cognitive challenges, such as difficulty with speech. All of the above were more related to persistent problems with smell but, surprisingly, not to the severity of their original case of COVID-19. This link is likely related to the brain's olfactory bulb, which processes smell—and also signals other portions of the brain that play a part in emotion, learning and memory.

"In some individuals, COVID-19 triggers a massive release of inflammatory signals that can lead to blood-brain barrier dysfunction, which can promote neuroinflammation and brain cell death," explains Rebecca Edelmayer, Ph.D., the Chicago-based senior director of scientific engagement for the Alzheimer's Association. "Other viral pandemics and epidemics—including the 1918 flu, SARS and MERS—have been connected to memory impairment, sleep disruption, anxiety, new onset seizures and delirium. These factors are all known to play a role in risk and progression of Alzheimer's and other dementia."

An illustration of a brain with covid-19 cells and fog over it
Credit: Getty Images / saemilee / bgblue

Other research presented at July's AAIC discovered that the biological markers of brain injury, brain inflammation and Alzheimer's disease correlate strongly with the presence of neurological symptoms in COVID-19 patients.

"These findings aren't necessarily surprising," Edelmayer adds. "Since the flu pandemic of 1917 and 1918, many flu-like viruses have been associated with brain disorders. The SARS-CoV-2 virus, which causes COVID-19, is also known to impact the brain and nervous system, as we've seen in infected people who have reported loss of smell and taste and 'brain fog.' Those symptoms are indicative that the virus may be impacting cells in the brain."

What's new here, Edelmayer points out, is that we're starting to see evidence build up that COVID-19 may be causing some lasting damage to the brain in older adults.

While the virus itself may only be active within the body for about two weeks, the impacts may be lifelong and potentially life-changing.

"The message is simple: don't get COVID-19; it is a preventable disease. The best way to do that is by getting vaccinated and by following public health guidance. If you had COVID-19, it doesn't mean you are destined to develop dementia or cognitive impairment, but it may increase your risk. If you had COVID-19 and are experiencing symptoms like memory loss or disorientation, talk to your doctor," Edelmayer says.

If you come down with COVID-19, it's important to continue to take care of your future self, especially your cardiovascular health.

"We know that what is good for your heart is also good for your brain, and COVID-19 may also have long-lasting effects on your cardiovascular health, which can increase risk for cognitive decline and dementia in the future," she adds.

COVID-19's effects on the brain also seem to vary with age, the researchers say. People in their 30s seem more likely to develop anxiety and depression, while seniors over 60 notice forgetfulness.

"Much more longitudinal research is needed to understand how COVID-19 affects the brain long-term," Edelmayer says. "This virus has not been around long enough for us to truly understand how infection may lead to cognitive decline and even risk for dementia."

These symptoms may improve over time, the scientists say, but the correlation is enough to make them suspicious that COVID-19 may speed up the biological process that leads to Alzheimer's disease later in life. For now, we can only wait and see, follow these brain-boosting strategies to reduce risk for dementia and get vaccinated to drastically lower the chances that you'll acquire coronavirus (if you aren't one of the more than 70% of American adults already vaccinated).