Longevity Briefs: More Evidence That Vaccines Can Reduce Dementia Risk

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Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

The problem:

Dementia is incurable, with most current treatments focusing on managing symptoms. For the moment, prevention might be more feasible than a cure – we know that lifestyle factors have a significant influence on dementia risk. Intriguingly, there is good evidence to suggest that many vaccines are also associated with a reduction in dementia risk. While there are many confounding factors to contend with (such as people who get vaccinated being generally better educated and caring more about health), there does also appear to be some inherent benefit to vaccination, possibly in the form of reduced inflammation in the brain. This study provides yet more compelling evidence that vaccination, in this case with the herpes zoster (shingles) vaccine, can reduce dementia risk.

The discovery:

Researchers conducted a large-scale study in Wales, using detailed electronic health records of 296,603 individuals born between 1925 and 1942. They took advantage of the fact that in 2013 in Wales, individuals born on or after September 2nd 1933 became eligible for the herpes zoster vaccine. This created a kind of natural randomized trial: individuals born just before the cutoff date were ineligible for the vaccine, yet are unlikely to be significantly different in other ways to those born just a few weeks later. The researchers followed participants for seven years after the start of the vaccination program, and performed extensive analyses to rule out potential confounding factors.

Probability of dementia diagnosis according to date of birth. Grey dots are placed at 10 week increments, and show the probability of dementia diagnosis for people born in that increment. The dotted red line marks birth date at which participants become eligible for vaccination, thus dots to the right of this line represent people who were born after the eligibility date and so were eligible for vaccination.
*A natural experiment on the effect of herpes zoster vaccination on dementia*

Unsurprisingly, people born earlier were more likely to develop dementia during the followup period, since they were older when the study began. However, as birth date reached the eligibility cutoff point, there was a statistically significant drop in dementia risk. The percentage of dementia cases among those born just after the cutoff date was 1.3 points lower than in those born just before the cutoff. Keep in mind that roughly half of those born after the cutoff date actually got vaccinated, so this drop in dementia risk does not represent the effect of the vaccine, but rather the effects of being eligible. The researchers estimate that the actual effect of the vaccine was a 3.5 percentage point drop, or a 20% reduction in risk relative to not receiving the vaccine.

The implications:

This study provides compelling evidence that the herpes zoster vaccine reduces risk of dementia. The beauty of this study design is that, by comparing individuals based on eligibility (instead of whether they actually took the vaccine or not), the potential confounding factors associated with willingness to receive the vaccine are eliminated. This makes it much more likely that the reduction in dementia risk was caused by the vaccine, rather than by people taking the vaccine being healthier in other ways as well.

This is far from being the first case of a vaccine appearing to reduce dementia risk. A previous observational study also suggested that vaccinations against herpes zoster, pneumococcus or tetanus and diphtheria with or without pertussis (Tdap/Td) are all associated with reductions in Alzheimer’s disease of 25-30%. Other studies suggest a similar risk reduction from influenza vaccines, which are taken by millions each year in the UK.