By Hendra Lukman 
An unusual public health policy implemented in Wales has yielded what may be the most compelling evidence to date suggesting that a widely administered vaccine can significantly lower the risk of developing dementia. A new study, spearheaded by researchers at Stanford Medicine and analyzing extensive health records of older adults in Wales, has uncovered a remarkable association: individuals who received the shingles vaccine were 20% less likely to be diagnosed with dementia over a subsequent seven-year period compared to their unvaccinated counterparts.
This groundbreaking research, slated for publication in the esteemed scientific journal Nature on April 2nd, lends substantial weight to a burgeoning scientific theory positing that viral infections impacting the nervous system can act as a catalyst for dementia. Should these findings be further corroborated, they signal a potential paradigm shift in dementia prevention, suggesting that a proactive intervention may already be within reach.
Understanding Shingles and its Viral Culprit
Shingles, a painful condition characterized by a distinctive rash, is caused by the varicella-zoster virus (VZV). This is the same virus responsible for chickenpox, a common childhood illness. Following an initial infection, VZV lies dormant within the body’s nerve cells, capable of reactivating later in life, particularly in individuals with weakened immune systems or advanced age. The reactivation manifests as shingles, often accompanied by significant discomfort and neurological symptoms.
The global burden of dementia is immense, affecting over 55 million people worldwide, with an estimated 10 million new cases emerging annually. For decades, the primary focus of dementia research has centered on the pathological hallmarks of Alzheimer’s disease, the most prevalent form of dementia, namely the accumulation of amyloid plaques and tau tangles in the brain. However, the persistent lack of significant breakthroughs in prevention and treatment has prompted a broader exploration of alternative avenues, including the potential role of viral infections.
Previous observational studies, drawing on health records, have hinted at a link between the shingles vaccine and reduced dementia rates. Nevertheless, these studies have been hampered by a significant confounding factor: a propensity for individuals who opt for vaccination to also exhibit healthier lifestyles overall. Behaviors such as maintaining a balanced diet and engaging in regular exercise are known to influence dementia risk, yet these crucial lifestyle elements are often not captured in standard health records, making it challenging to isolate the vaccine’s independent effect.
Dr. Pascal Geldsetzer, an assistant professor of medicine and senior author of the new study, articulated this challenge: "All these associational studies suffer from the basic problem that people who get vaccinated have different health behaviors than those who don’t. In general, they’re seen as not being solid enough evidence to make any recommendations on."
A "Natural Experiment" in Public Health Policy
The current study’s strength lies in its astute identification of a fortuitous "natural experiment" presented by the rollout of the shingles vaccine in Wales. This policy, designed to manage the distribution of a live-attenuated (weakened) form of the VZV vaccine, inadvertently created a near-ideal scenario for researchers to circumvent the biases inherent in observational studies.
The vaccination program commenced on September 1, 2013. Eligibility was strictly determined by age on that specific date. Individuals who were 79 years old on September 1, 2013, were granted a one-year window to receive the vaccine. The following year, those who had turned 78 on the initial date became eligible for a year, and so forth. Crucially, individuals aged 80 or older on September 1, 2013, were permanently excluded from this initial vaccination phase, meaning they would never become eligible under this particular program.
These age-based rationing rules, implemented due to the limited supply of the vaccine, created a distinct boundary. A mere difference of a few days in a person’s birthday could determine their access to the vaccine. By meticulously comparing individuals who turned 80 just before September 1, 2013, with those who turned 80 just after, the researchers were able to isolate the impact of vaccine eligibility.
"The circumstances, well-documented in the country’s health records, were about as close to a randomized controlled trial as you could get without conducting one," Dr. Geldsetzer noted.
The research team meticulously examined the health records of over 280,000 older adults, aged between 71 and 88, who had no prior diagnosis of dementia at the program’s inception. Their analysis honed in on individuals situated at the eligibility threshold – those who celebrated their 80th birthday in the week preceding the September 1st cutoff, and those who did so in the week immediately following.
"We know that if you take a thousand people at random born in one week and a thousand people at random born a week later, there shouldn’t be anything different about them on average," Dr. Geldsetzer explained. "They are similar to each other apart from this tiny difference in age." The expectation was that similar proportions of both groups would have desired the vaccine, but only those who met the age criteria were permitted to receive it.
"What makes the study so powerful is that it’s essentially like a randomized trial with a control group – those a little bit too old to be eligible for the vaccine – and an intervention group – those just young enough to be eligible," Dr. Geldsetzer emphasized.
Unveiling Protection Against Dementia
Over a seven-year period, the researchers meticulously tracked the health outcomes of individuals who were close in age but differed in their eligibility for the shingles vaccine. By accounting for actual vaccination rates – approximately half of those eligible received the vaccine, while virtually none of the ineligible group did – they could deduce the specific effects of vaccination.
As anticipated, the vaccine demonstrated efficacy in reducing the incidence of shingles by approximately 37% among those who received it during the study period. This finding aligned with previously established effectiveness rates from clinical trials of the vaccine, though it is important to note that the effectiveness of the live-attenuated vaccine can wane over time.
By 2020, when the study participants had reached the ages of 86 and 87, one in eight older adults in the overall cohort had been diagnosed with dementia. However, a significant divergence emerged: those who had received the shingles vaccine exhibited a 20% lower likelihood of developing dementia compared to their unvaccinated peers.
"It was a really striking finding," Dr. Geldsetzer remarked. "This huge protective signal was there, any which way you looked at the data."
The research team conducted a comprehensive search for other potential factors that might have influenced dementia risk, scrutinizing a wide array of variables. They found that the two groups – eligible and ineligible for the vaccine – were remarkably similar across all measured characteristics. For instance, there was no discernible difference in educational attainment. Those who were eligible were not more inclined to receive other vaccinations or preventive treatments, nor were they less likely to be diagnosed with other common health conditions such as diabetes, heart disease, or cancer. The only statistically significant difference observed was the reduced incidence of dementia diagnoses in the vaccinated group.
"Because of the unique way in which the vaccine was rolled out, bias in the analysis is much less likely than would usually be the case," Dr. Geldsetzer stated, highlighting the robust nature of their findings.
To further validate their results, the team employed alternative analytical approaches, including examining different age ranges and focusing solely on dementia-attributed deaths. In every instance, the association between shingles vaccination and reduced dementia rates remained consistent and robust.
"The signal in our data was so strong, so clear and so persistent," he affirmed.
Differential Impact and Future Directions
An intriguing secondary finding emerged from the study: the protective effect of the shingles vaccine against dementia appeared to be significantly more pronounced in women than in men. Dr. Geldsetzer suggested that this could be attributed to sex-based differences in immune responses or variations in the underlying mechanisms of dementia development. For example, women typically exhibit higher antibody responses to vaccinations, and shingles itself is more prevalent in women.
The precise mechanism by which the vaccine might confer protection against dementia remains an open question. Potential explanations include a general boost to the immune system, a specific reduction in VZV reactivation, or other as-yet-undetermined pathways.
Furthermore, the study did not assess the impact of newer shingles vaccines, which utilize recombinant technology and contain specific viral proteins for enhanced efficacy in preventing shingles. It is yet unknown whether these newer formulations might offer similar or even greater protection against dementia.
Dr. Geldsetzer expressed optimism that these findings will stimulate increased investment in this critical area of research. "At least investing a subset of our resources into investigating these pathways could lead to breakthroughs in terms of treatment and prevention," he urged.
The research team has already taken steps to broaden their investigation. Over the past two years, they have successfully replicated the Welsh findings by analyzing health records from other countries, including England, Australia, New Zealand, and Canada, which implemented similar vaccine rollout strategies. "We just keep seeing this strong protective signal for dementia in dataset after dataset," Dr. Geldsetzer reported.
Despite the compelling evidence from these observational studies, Dr. Geldsetzer’s ultimate goal is to conduct a large-scale, randomized controlled trial. Such a trial would provide the definitive proof of causality by randomly assigning participants to receive either the live-attenuated shingles vaccine or a placebo.
"It would be a very simple, pragmatic trial because we have a one-off intervention that we know is safe," he explained. Securing philanthropic funding for this trial is a current priority, as the live-attenuated vaccine is no longer in active production by pharmaceutical companies.
The potential timeline for observing results in such a trial is also encouraging. Dr. Geldsetzer pointed to data from the Welsh study, illustrating that the divergence in dementia rates between the eligible and ineligible groups began to become apparent within approximately eighteen months.
The study benefited from the contributions of a researcher from the Vienna University of Economics and Business and received funding from The Phil & Penny Knight Initiative for Brain Resilience, the National Institute on Aging (grant R01AG084535), the National Institute of Allergy and Infectious Diseases (grant DP2AI171011), and the Chan Zuckerberg Biohub. This collaborative effort underscores the growing recognition of the potential link between viral infections, immune response, and neurodegenerative diseases, opening a new frontier in the fight against dementia.