Roughly 5% of Covid-19 patients - 27 million people - have reported a loss of smell lasting for more than six months.
Researchers and health experts are now seeing strong links between this symptom and cognitive decline.
Alzheimer’s disease is synonymous with failing cognition.
Preliminary findings presented at the Alzheimer’s Association International Conference in San Diego suggest there may be a link between ongoing loss of smell and an increased Alzheimer's disease risk.
The research further investigates the big question of whether Covid-related loss of smell may be associated with cognitive decline, and what this could mean for Alzheimer's disease risk.
Does Covid increase Alzheimer's risk?
One of the biggest symptoms caused by Covid infection could be affecting a person’s memory and concentration.
New evidence suggests that loss of smell has been found to cause long-term neurological issues.
Scientists have highlighted a link between cases of Covid-19 that include neurological symptoms, with biological markers of Alzheimer’s disease.
The research warns that persistent loss of smell after a Covid infection could be a predictor of lingering cognitive issues, even more than disease severity.
In the study, researchers in the US followed 766 participants aged over 60 for three to six months after their Covid infection.
Participants took a PCR test at a testing clinic in Argentina, with nearly 90% testing positive for the viral disease.
The researchers collected data on disease severity and provided a sniff test for participants, and a series of cognitive assessments, three months after the PCR.
The team found that two-thirds of the people who tested positive for Covid-19 experienced some form of memory impairment.
For half of that group, the memory impairment was severe enough that it interfered with their daily lives.
Speaking about these new findings, Dr Sara Imarisio, head of research at Alzheimer’s Research UK, said: “With one in three people born today expected to develop dementia in their lifetime, there is a pressing need to identify risk factors for the condition as well as developing treatments for people with the disease.
“Relatively little is known about the long-term impacts of Covid-19 on brain health, and Alzheimer’s Research UK remains committed to monitoring the emerging evidence in this field.
“Previous research, before the Covid-19 pandemic, has highlighted a potential link between changes to people’s sense of smell and dementia risk, and some researchers have explored using smell tests as a way to identify people in the very earliest stages of diseases like Alzheimer’s.”
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”.
She added: “We know inflammation is part of the neurodegenerative process in diseases like Alzheimer’s but we need to dig deeper into exactly how they are connected.”
Alzheimer's disease is thought to be caused by the abnormal build-up of proteins in and around brain cells.
Previous research has also found that inflammation in the brain drives the progression of the disease.
“Studies like these are good for highlighting links, but we need research to explore the mechanisms between Covid-19 and long-term memory and thinking changes. This research didn’t look at whether people went on to develop dementia and, as the diseases like Alzheimer’s develop in the brain over many years, it will require longer follow-up to reveal whether these changes are linked to dementia.
“We are just beginning to develop an understanding of the long-term consequence of Covid-19 infections, and it will take much larger studies to understand if there are specific factors that could be relevant to dementia risk.
“Changes to our sense of smell are not necessarily an indication of future memory problems or a consequence of Covid-19.
“If anyone is worried about their memory and thinking, or long-term effects of Covid-19, they should speak to their doctor.”
