A recent study of more than 6 million people 65 and older found that seniors who had Covid-19 had a substantially higher risk of being diagnosed with Alzheimer’s disease within a year.
The study does not show that Covid-19 causes Alzheimer’s, but it adds to the growing body of research drawing links between coronavirus infection and cognitive function.
“In the Alzheimer’s disease brain, the pathology starts to build up about 20 years before the symptoms begin,” said Dr. David Holtzman, a neurologist who leads a research lab focused on Alzheimer’s disease at the Washington University School of Medicine in St. Louis. People would have to be followed for decades after a Covid-19 infection to prove it as a cause, he said.
Instead, a Covid-19 infection could cause inflammation that may exacerbate changes that are already happening in the brain, experts say.
“The brain has its own immune response to the pathology that’s involved in [Alzheimer’s] disease progressing,” said Holtzman, who was not part of the new study. “When there are other things that cause inflammation that are in the body that can affect the brain, likely what happens is that can even amplify the process that’s already going on.”
Other viruses can cause similar inflammation, experts say.
Covid “is another one of the many dozen potential risk factors that I talked about with my patients,” said Dr. Richard Isaacson, a neurologist and director of Florida Atlantic Unviersity’s Center for Brain Health. He also was not involved in the new study but is a researcher focused on risk prevention for Alzheimer’s disease.
“I tell people to get a shingles vaccine. I tell people to get their annual flu and Pneumovax,” and to exercise and eat a brain-healthy diet.
Still, “when there’s smoke, there’s fire at some point,” he said. “I really believe that this is something to at least pay attention to.”
The latest study, published last week in the Journal of Alzheimer’s Disease, found that there were about seven new diagnoses of Alzheimer’s disease for every 1,000 seniors who had a documented case of Covid-19 in the past year, compared with about five new diagnoses for every 1,000 who did not.
Heather Snyder, vice president of medical and scientific relations at the Alzheimer’s Association, notes that broader implications of the pandemic could have played into the study’s findings.
“The pandemic presented serious delays for individuals seeking out medical diagnoses like Alzheimer’s, meaning these results could be driven by those who already had Alzheimer’s when they were infected but had not yet sought out a formal diagnosis,” she said.
The study authors, along with Snyder and other experts, also identify this work as a call for more research on the underlying mechanisms of Alzheimer’s disease that might explain the association.
In the new study, the diagnosis of Alzheimer’s was “mostly tentative,” said Dr. Eliezer Masliah, director of the Division of Neuroscience at the National Institutes of Health’s National Institute on Aging.
Masliah, who was not involved in the study, said that there’s evidence that Covid-19 might “trigger cognitive impairment,” but there are new ways to confirm the link to Alzheimer’s specifically.
One next step would be to follow people at risk for Alzheimer’s after a Covid-19 infection long-term to track biomarkers found in the blood and brain scans.
“In the next couple of years, we’re going to have a lot of very important information,” Masliah said. And it’s an “extremely important problem” to watch, given the scale of disease.
“Imagine how many millions of people over the age of 60 or 65, like myself, have had Covid. Say 5% of them or 10% of them or even 1% of them are at risk,” he said.
“Wow. We’re looking at a lot of people in the next few years that might add to the already very large epidemic of Alzheimer’s disease that we have.”
Get CNN Health's weekly newsletter
Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.
About 6.5 million people over the age of 65 are living with Alzheimer’s, according to estimates from the Alzheimer’s Association. And it was the seventh leading cause of death in the US in 2020, according to data from the US Centers for Disease Control and Prevention.
“Alzheimer’s disease is a serious and challenging disease, and we thought we had turned some of the tide on it by reducing general risk factors such as hypertension, heart disease, obesity and a sedentary lifestyle,” said Dr. Pamela Davis, a research professor at Case Western Reserve University and co-author of the new study.
“Now, so many people in the US have had Covid, and the long-term consequences of Covid are still emerging. It is important to continue to monitor the impact of this disease on future disability.”