One of the stranger symptoms of Covid — the loss of the sense of smell — is a symptom that, well before the pandemic, was considered to be a warning sign for dementia.

The big question for researchers now is whether Covid-related loss of smell might also be associated with cognitive decline. Around 5 percent of Covid patients worldwide — some 27 million people — have reported loss of smell lasting more than six months.

New preliminary findings presented Sunday at the Alzheimer’s Association International Conference in San Diego suggest there may be a link, though experts caution that more research is needed.

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Previous research has found that some Covid patients go on to develop cognitive impairment after their infection. In the new study — which has not been published in a peer-reviewed journal —  researchers in Argentina found that loss of smell during Covid may be a stronger predictor of cognitive decline, regardless of severity of disease.

“Our data strongly suggest that adults over 60 years of age are more vulnerable to cognitive impairment post-Covid if they had a smell dysfunction, regardless of the severity of the Covid,” said study co-author Gabriela Gonzalez-Aleman, a professor at Pontificia Universidad Catolica Argentina in Buenos Aires, adding that it’s too soon to tell if the cognitive impairment is permanent.

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The study tracked 766 adults ages 55 to 95 for a year after their infection. Nearly 90 percent had a confirmed case of Covid and all completed regular physical, cognitive and neuropsychiatric tests over the course of one year.

Two-thirds of those infected had some type of cognitive impairment at the end of that year. In half of the participants, the impairment was severe.

The researchers did not have hard data on the state of cognitive function of the patients before contracting Covid in order to compare with the findings at the end, but they did ask participants’ families about their cognitive function before infection and did not include people who had clear cognitive impairment before the study.

According to Jonas Olofsson, a professor of psychology at the University of Stockholm who studies the link between sense of smell and dementia risk — and was not involved in the new research, smell loss is a well-established precursor of cognitive decline. It’s also well established that Covid can lead to lasting loss of smell, he said.

“The question is whether those two lines of research intersect,” Olofsson said. “This study is quite tantalizing, although the information that I have seen so far does not allow for any strong conclusions. ”

The smell-brain connection

According to Dr. Claire Sexton, senior director of scientific programs and outreach at the Alzheimer’s Association, “Loss of smell is a signal of an inflammatory response in the brain.”

“We know inflammation is part of the neurodegenerative process in diseases like Alzheimer’s,” Sexton said. But we need to dig deeper into exactly how they are connected.”

A separate study — not related to Covid — published last Thursday in the journal Alzheimer’s & Dementia probes that connection further. Researchers at the University of Chicago found that not only can a decline in sense of smell over time predict loss of cognitive function, loss of the sense of smell can also be a warning sign of structural changes in regions of the brain important in Alzheimer’s disease and dementia.