Alzheimer’s disease is the most common form of dementia, affecting more than 10 million people in Europe. The thought of developing dementia fills most people with dread. The disease imposes a heavy burden on individuals, their families and health systems. Older people, especially women, are at higher risk of developing dementia. It’s no wonder researchers have been working on new treatments and vaccines.
However, 40 years of intensive studies have yet to deliver an effective therapy, with fears that the field may have hit a dead end. Things are a little more promising in the field of vaccine research with reports of up to nine potential Alzheimer’s vaccines in trials. However, most are in the earliest stages of development and ‒ if they ever meet the strict safety and efficacy standards required ‒ it will be several years from now before they ever reach the public.
Breaking news on old vaccines
But all is not lost. A striking new paper from the Journal of Alzheimer’s Disease raises a new prospect: existing vaccines may significantly reduce the risk of dementia simply by keeping people in good overall health for longer.
The US study shows that people who have received at least one influenza vaccine were 40% less likely to develop Alzheimer’s disease over the following four years. This is just an association ‒ scientists have not pinpointed an exact mechanism to explain what’s going on ‒ but it’s an eye-catching paper nonetheless.
For a start, it’s a huge study. Observing a correlation between two things in a small group wouldn’t mean much, but this one includes data from around 2 million people.
A second potential problem for research of this kind is the risk of confounders ‒ other factors that might have influenced the result. For example, if you found that people who go on holiday frequently are less likely to have Alzheimer’s, it would be tempting to book a week off and head to the airport.
But we also know that poor people are at higher risk of dementia and that poor people go on fewer holidays. Maybe living in bad housing or having a bad diet all year have more to do with dementia than whether you spend a fortnight in the sun. So, let’s not get too carried away (unless you just want an excuse to hit the beach.)
A third danger is that researchers go to the trouble of comparing two groups of people, giving one a vaccine or treatment and giving the other nothing (or a placebo) but the two groups are not similar enough. The results might look good, but would be undermined if by comparing two groups with different characteristics. For example, if the typical age of people in the first group is 50 and the typical person in the second group is 70, you’ve got a big problem. Likewise, having more women than men in an Alzheimer’s study will skew the results.
Comparing like with like
However, the new study linking flu vaccine to lower Alzheimer’s risk avoids these traps. It’s a matched study, meaning they took data from nearly 1 million people vaccinated against flu and compared it with nearly 1 million other unvaccinated people, matching them by age, race, gender and where they lived. They also accounted for medication usage and comorbidities to ensure that, for example, they didn’t have a disproportionate number of people with diabetes or high blood pressure (both risk factors for dementia) in either group.
Looking back at 10 years (from 2009 to 2019) of health insurance claims data from these pairs of people aged over 65 years, they were able to study the impact of flu vaccination on subsequent Alzheimer’s diagnosis. And the results were clear: flu vaccines cut your risk of dementia. In fact, the strength of the protective effect increased with the number of years the person received an annual flu vaccine.
This is not entirely shocking, but it is a very strong conclusion to a very large study. An earlier paper by scientists in Taiwan, looking at the risk of dementia among a smaller number of army veterans, also found that vaccinated people reduce their risk of cognitive decline.
So what’s going on?
To be frank, that’s not very clear. It may be that this phenomenon is not limited to flu vaccines alone. Other vaccines, including the BCG tuberculosis vaccine and the shingles vaccine have also been linked to lower rates of dementia. Similarly, measles vaccines have been shown to reduce the rates of other infectious illnesses. It may be that vaccines help to avoid diseases, such as flu or shingles, that would otherwise trigger illnesses or reduce the body’s capacity to respond to other infections.
Professor Paul Schulz, lead author of the new study on flu vaccines and dementia suggests infections could be accelerating cognitive decline. ‘Since there is evidence that several vaccines may protect from Alzheimer’s disease, we are thinking that it isn’t a specific effect of the flu vaccine,’ he said. ‘Instead, we believe that the immune system is complex, and some alterations, such as pneumonia, may activate it in a way that makes Alzheimer’s worse. But other things that activate the immune system may do so in a different way – one that protects from Alzheimer’s disease. Clearly, we have more to learn about how the immune system worsens or improves outcomes with this disease.’
Future research will look at whether flu vaccination is associated with the rate at which Alzheimer’s disease progresses in patients who already have the early stages of dementia. The researchers also want to collect more information on whether there could be a similar association between COVID-19 and the risk of Alzheimer’s disease. Or, to be more optimistic about it, they want to know whether being vaccinated against COVID-19 could help some people avoid or delay the onset of cognitive decline.
In the meantime, it seems sensible to continue having vaccines recommended by your doctor. And to reduce your risk of dementia by not smoking and by doing what you can to reduce the risk of diabetes and obesity.