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Long COVID-19 can cause memory problems. Does it raise Alzheimer's risk?

New research shows how the coronavirus continues to affect the brain, even long after the virus is gone.

When Rachel Bean tested positive for COVID-19 on May 1, 2020, her case was labeled asymptomatic. She had been short of breath for a couple of weeks before but passed it off as pandemic anxiety and getting used to wearing a mask. But throughout the coming months, the symptoms rolled in.

By July, despite testing negative for the virus, Bean, 34, of Minneapolis, was struggling to eat after losing her sense of taste and smell. She had heart palpitations. The symptoms came and went, and by January, Bean had to take a three-month leave from her job at a harm reduction housing program. She enrolled in a post-COVID-19 research clinic, where she worked with a bevy of therapists to regain some of the fine motor skills she had lost and underwent a battery of cognitive tests that revealed that she was processing information more slowly than before.

Now, more than a year after her mild case, Bean still makes mistakes when she's driving. She forgets names and occasionally finds herself putting frozen food in the kitchen cabinet. When she speaks, she often has to ask people to rattle off the names of everyday items that she struggles to remember.

The symptoms are similar to those of a disease that's been around much longer than COVID-19: Alzheimer's.

Researchers are just beginning to piece together how COVID-19 affects the body long-term, but it's clear that the disease causes lasting cognitive impairment in some people, including those who have had mild cases. Whether the changes have any links to Alzheimer's is a topic of research presented Thursday at the Alzheimer's Association International Conference in Denver. One study found that loss of smell appears to be tied to symptoms related to cognitive function. Another found that some of the same biomarkers that indicate brain damage and Alzheimer's disease also appear in COVID-19 patients with memory loss.

"We don't know if COVID-19 is causing Alzheimer's disease," said Heather Snyder, vice president of medical and scientific relations at the Alzheimer's Association. "What we do see is that some people who have had COVID-19 and experience persistent changes, like loss of smell, also have persistent changes in their memory and markers of brain disease and injury. We need to continue to follow these patients to determine what the long-term impact looks like and whether it worsens, stays the same or gets better and why."

Long-term cognitive symptoms from COVID-19 appear to differ with age. Depression, anxiety and sleep trouble are more common among young people, and memory loss and speech impairment are more common in people over age 65, said Dr. Gabriel de Erausquin, director of the Laboratory of Brain Development, Modulation and Repair at the Glenn Biggs Institute of Alzheimer's and Neurodegenerative Disorders in San Antonio.

According to one of the new studies presented Thursday, people who lose their sense of smell are more likely to experience cognitive impairment, which may show up as memory loss or troubles with speech. Loss of smell also predicts the severity of brain changes and cognitive decline in people with Alzheimer's disease.

"If you have a loss of smell, you won't necessarily have cognitive impairment, but if you do, the more severe the loss of smell, the more severe the loss of memory will be," said de Erausquin, who is leading the research on a growing group of more than 300 Argentines ages 60 and up who have had COVID-19.

The connection likely has to do with a part of the brain called the olfactory bulb, which processes sense of smell. The olfactory bulb also sends signals to other parts of the brain that play a role in emotion, memory and learning. Past research has shown that this is the point of entry that allows the coronavirus to infiltrate the brain, which is why people commonly lose their sense of smell. However, it's also possible that the virus may not have to reach the brain to affect the olfactory bulb. Instead, memory loss or speech impairment may be caused by the body's inflammatory response to the virus.

Past research also showed that people who were sicker with COVID-19 were more at risk for cognitive impairment, but only 10% of patients in the Argentine cohort were hospitalized, and 50% had cognitive impairment, suggesting that the severity of the disease at the start doesn't appear to determine long-term effects. And in another new study, published Thursday in JAMA Network Open, Norwegian researchers found that 12% of people reported continual issues with concentration eight months after they had COVID-19. Eleven percent had ongoing memory problems.

"The actual disease typically lasts about two weeks, but after the virus is gone, people aren't recovered," said Feixiong Cheng, assistant staff at the Cleveland Clinic's Genomic Medicine Institute, who wasn't involved with the new research. "COVID-19 sets off biological processes that can produce long-term effects even in people who had mild or no symptoms at the start."

Another piece of preliminary research presented at the conference by New York University physicians focused on biological markers in the blood. They found that COVID-19 patients who had cognitive impairment, most commonly confusion, had some of the same biomarkers that indicate brain injury, neuroinflammation and Alzheimer's disease. The researchers said they believe the biomarkers may be tied to inflammation in the blood-brain barrier caused by COVID-19; they said in a news release that more research is needed to determine whether or not those patients are at higher risk for developing Alzheimer's disease down the road.

Cheng led a study published last month in the journal Alzheimer's Research & Therapy that also linked biomarkers indicative of Alzheimer's disease and dementia to those found in recovered COVID-19 patients who experienced cognitive impairment.

"COVID-19 does increase the probability of having cognitive impairment, but for all we know, people may improve over time," de Erausquin said. Even so, "that coincidence makes us suspicious that COVID-19 may be accelerating the biological brain process that leads to Alzheimer's disease over the course of 10 to 20 years."

The impacts on the brain are concerning, given the staggering number of COVID-19 cases in the world.

"The majority of people with COVID-19 do not end up in the ICU, but a lot of people still have cognitive impairment that greatly impacts their lives, and that's very worrying," said Dr. Wes Ely, co-director of the Critical Illness, Brain Dysfunction and Survivorship Center at Vanderbilt University and the Veterans Administration. "It creates a disability in patients who never got very sick."

This story was originally published on NBC News.