A new study published in the scientific journal BMJ Open has found that herpes simplex virus type 1 (HSV-1), which causes cold sores, is associated with an increased risk of Alzheimer’s disease.
However, experts emphasize that the findings show correlation, not causation, and more research is needed.
The study, conducted by researchers at Gilead Sciences Inc., analyzed health insurance data from 344,628 U.S. adults aged 50 and older diagnosed with Alzheimer’s or related dementia between 2006 and 2021, each matched with a control of similar age, sex, and region without neurological disorders.
Of those with Alzheimer’s, 0.44% had a prior HSV-1 diagnosis, compared to 0.24% of controls, suggesting an 80% higher relative risk for Alzheimer’s among those with HSV-1. The absolute numbers, however, remain small. Additionally, individuals treated with antiviral medication for HSV-1 showed a 17% lower risk of developing Alzheimer’s compared to untreated counterparts.
“Despite the large sample size, this research has limitations partly due to only using health records and administrative claims data,” said Dr. Sheona Scales, Director of Research at Alzheimer’s Research UK, in a statement to the Science Media Centre. “Most people infected with HSV-1 don’t have any symptoms, so some infections might not have been recorded.”
Dr. David Vickers of the University of Calgary was more critical, stating, “This pharma-funded research exaggerates the role of HSV-1, failing to appreciate its absence in 99.56 percent of [Alzheimer’s disease] cases.”
The World Health Organization estimates two-thirds of people under 50 globally carry HSV-1, a neurotropic virus that can infect nerve cells and often lies dormant after initial infection, sometimes reactivating to cause cold sores.
Previous studies have detected HSV-1 DNA in postmortem Alzheimer’s brains and shown it can trigger amyloid-beta plaque accumulation, a hallmark of the disease, in lab settings. A 2024 Journal of Virology study found HSV-1 can enter mouse brains, causing persistent inflammation, noting evidence for its role in Alzheimer’s as “overwhelming.”
“We’re not saying viruses explain everything. But they may be central to it. This is no longer a fringe theory – it’s the next phase of Alzheimer’s research, and we’re pursuing it,” Dr. Bryce Vissel of St. Vincent’s Hospital Sydney told the Australian Science Media Centre.
However, Professor Tara Spires-Jones of the University of Edinburgh cautioned, “It is important to note that HSV-1 infection, which is extremely common in the population, is by no means a guarantee that someone will develop Alzheimer’s.”
The study’s reliance on insurance data raises concerns, as HSV-1 is often underdiagnosed, potentially skewing results. It also lacks data on infection frequency or severity, which could influence risk.
“The study’s data source makes its findings ungeneralisable, and it overstates a minor infection as a ‘public health priority’ to justify unnecessary treatment,” Vickers added.
Other factors, like genetics, lifestyle, or healthcare access, may also contribute to Alzheimer’s risk among HSV-1 carriers.
Some experts see potential in antiviral treatments.
“With many GPs and the population being unaware of the dementia related benefits of treating HSV infections and preventing VZV activation through vaccination, it is time to call for actions informing those working in primary care as well as the population at large,” said Professor Cornelia van Duijn of the University of Oxford.
Dr. Richard Oakley of Alzheimer’s Society advised, “If you are worried about a cold sore or your general health, be sure to seek the appropriate help from a health professional.”
“More research is needed to understand the best way to protect our brains from Alzheimer’s disease as we age,” Spires-Jones said. For now, experts recommend focusing on proven brain health strategies like exercise, sleep, and a balanced diet.