Scientists have issued a warning that persistent brain fog, headaches, and alterations in smell or taste following a Covid-19 infection could indicate an elevated risk of developing Alzheimer's disease later in life. This concern stems from findings by US researchers who analyzed blood samples from over 225 long-Covid patients and discovered significantly increased levels of tau, a protein strongly associated with Alzheimer's and other forms of dementia. Abnormal clumps of tau are known to form tangles within the brain's nerve cells, disrupting communication and driving the memory loss and cognitive decline characteristic of Alzheimer's, which is the leading cause of dementia.

Dr. Benjamin Luft, an infectious disease expert and lead author of the study, emphasized the potential long-term consequences of Covid-19, stating that these impacts could manifest years after the initial infection, leading to chronic illnesses, including neurocognitive problems resembling those seen in Alzheimer's disease. He highlighted the practical implications for developing effective vaccines and therapies to prevent acute infection from embedding itself and causing long-term disease. The researchers published their findings in the journal eBioMedicine, concluding that individuals who experience persistent symptoms after Covid may face an increased risk of future neurodegenerative disease.

The study involved analyzing blood samples from 227 participants in the World Trade Center Health Program, a long-running cohort of 9/11 first responders. Samples were taken both before they contracted Covid and again, on average, 2.2 years post-infection. The investigation focused on pTau-181, a specific abnormal subtype of tau robustly linked to Alzheimer's disease. Results showed that participants experiencing neurological symptoms such as headaches, vertigo, or brain fog demonstrated an almost 60 percent rise in blood tau levels after Covid. Furthermore, those whose cognitive symptoms persisted for more than 18 months exhibited significantly higher levels of tau biomarkers compared to those whose symptoms resolved sooner.

Professor Sean Clouston, a preventive health expert and study co-author, explained that elevated tau in the blood is a recognized biomarker for lasting brain damage. He suggested that these findings imply long Covid could worsen over time, leading to progressively intensifying neurological symptoms or cognitive difficulties. However, he also cautioned that it is not yet known whether the observed increase in tau follows the same biological trajectory as seen in individuals who ultimately develop Alzheimer's or related diseases. The research team compared their findings to a control group of 227 World Trade Center responders who either did not contract Covid or had the infection without developing long-term symptoms, also known as neurological post-acute sequelae of Covid (N-PASC). This control group showed no significant rise in blood tau levels.

The researchers are now pursuing further investigations to determine if elevated tau in long-Covid patients is directly linked to the development of neurodegenerative diseases like Alzheimer's. Their next step involves validating these findings using neuroimaging to ascertain whether rising plasma tau levels correlate with increased tau accumulation in the brain. The authors noted a limitation that their cohort, consisting of essential workers, might not be representative of the wider population due to potentially greater environmental exposure to the virus. Despite this, they underscored the study's significance as one of the first to suggest that a virus can contribute to abnormal tau production over time, offering crucial insights into the biological pathways leading to neurodegenerative disease.

According to the NHS, long Covid, or post-Covid syndrome, is defined by symptoms persisting for more than 12 weeks after infection. NHS England survey data indicates that nearly one in ten people believe they may have long Covid. Figures from the Office for National Statistics reveal that approximately 3.3 percent of people in England and Scotland, equivalent to about two million individuals, were experiencing long Covid symptoms, with 71 percent reporting symptoms lasting at least a year, and over half indicating their symptoms had persisted for two years or longer. Alzheimer's disease currently affects around 982,000 people in the UK, a figure projected to increase to 1.4 million by 2040. Its early symptoms typically include memory problems, difficulties with thinking and reasoning, and language impairment, which progressively worsen over time.