A study using the electronic health records of more than 6 million Americans over age 65 found that those who had COVID-19 ran a greater risk of receiving a new diagnosis of Alzheimer’s disease within a year.

The study, led by researchers at Case Western Reserve University School of Medicine and published in the Journal of Alzheimer’s Disease, does not show that COVID-19 causes Alzheimer’s, but it adds to a growing body of work suggesting links between the two.

The results suggest researchers should be tracking older patients who recover from COVID to see if they go on to show signs of memory loss, declining brain function or Alzheimer’s disease. The study found that for every 1,000 seniors with COVID-19, seven will be diagnosed with Alzheimer’s within a year, slightly above the five-in-a-thousand diagnosis rate for seniors who did not have COVID.

“We know that COVID can affect the brain, but I don’t think anyone had looked at new diagnoses of Alzheimer’s,” said Pamela Davis, one of the study’s co-authors and a research professor at Case Western Reserve University School of Medicine. Colleague Rong Xu said she had expected to see some increase among seniors sickened by COVID, but was surprised “by the extent of the increase and how rapidly it occurred.”

The study, though “important and useful” was “limited,” said Gabriel de Erausquin, director of the Laboratory of Brain Development, Modulation and Repair at University of Texas Health San Antonio, who was not involved in the research.

He cautioned that a diagnosis of Alzheimer’s disease is not necessarily confirmation of the disease. Doctors sometimes diagnose Alzheimer’s based on changes in behavior, or responses to a memory test. These are considered less accurate than imaging or spinal fluid tests that measure two types of proteins, beta-amyloid and phosphorylated tau, which accumulate abnormally in the brains of people with Alzheimer’s. Brain scans that look for structural changes, such as the shrinking of certain regions, are another more accurate indicator.

“You have people who very much look like they have Alzheimer’s, but they do not have Alzheimer’s,” said de Erausquin, adding, “People who have long COVID are at risk of being diagnosed with Alzheimer’s disease.”

In their paper, the Case Western Reserve researchers acknowledged that the study was limited by the potential for inaccurate diagnoses of Alzheimer’s.

Davis, however, said she thinks it is unlikely doctors would mistake long COVID for Alzheimer’s.

“To label somebody with Alzheimer’s disease is different from the brain fog and the confusion associated with long COVID,” she said. “When a physician makes a diagnosis of Alzheimer’s disease, it represents a substantial cognitive impairment.”

The Case Western Reserve study used a database representing patients from almost 70 health care centers across the country. The scientists focused on patients over 65 who made a medical visit during a 15-month period from Feb. 2, 2020, to May 30, 2021, dividing them into two groups: those diagnosed with COVID-19, and those with no COVID diagnosis.

Through much of the study period, the electronic record system did not include a code for long COVID, leaving open the possibility that patients who had continuing health problems after their bout with COVID may have been misdiagnosed with Alzheimer’s.

Still, the new study adds to a growing body of research examining the relationship between COVID-19 and Alzheimer’s disease. An earlier study by some of the same researchers at Case Western Reserve examined the electronic health records of almost 62 million Americans over age 18 and found that patients with dementia had a significantly higher risk for COVID-19 than patients without dementia.

Thomas Wisniewski, director of the NYU Langone Alzheimer’s Disease Center, raised the possibility that some patients may already be on their way to Alzheimer’s when they get COVID-19, causing their brains “to take a second hit.”

He said scientists need to look at whether Alzheimer’s biomarkers that appear in COVID-19 patients “stay increased over time, and do they result in a faster disease trajectory?”

Scientists at Columbia University Vagelos College of Physicians and Surgeons studied the brains of a small number of deceased COVID-19 patients and found they had defective receptors considered to be an indicator of Alzheimer’s disease.

A second study looked at the serum, a component of blood, from hospitalized COVID-19 patients with no prior history of dementia and they had developed markers of neurodegenerative disease at comparable levels to those found in Alzheimer’s disease patients.

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