Flu vaccination linked to 40% reduced risk of Alzheimer’s disease

Which seems to me to be, unambiguously, you should absolutely be religious about getting your flu shot every year. The vaccine is known to be safe and with few side effects (or at least, safer than the actual flu, which is the bar that needs to be cleared.)

And even if it hasn't been conclusively proven that vaccines can avert Alzheimer's, you're at least putting yourself in the best possible cohort, right? Anyone familiar with Bayes' Theorem should be absolutely on board with this.

> Among developed countries, Japan has the lowest prevalence of both dementia in general and Alzheimer's disease in particular.

The COVID vaccines are also "known" to be safe and with few side effects, by the sort of public health people who like to make such claims. That doesn't mean anything.

Here's the problem with flu vaccines. They do not correlate with any reduction in mortality attributed to flu. Every trial indicates effectiveness, yet despite hugely increased number of flu vaccines over the years, deaths due to flu do not move. A rational person should wonder what this implies about the reliability of vaccine trials.

> The burden of dementia is increasing exponentially especially in Asia-Pacific region, where more than 60% of the population reside [34]. The prevalence of dementia seems to be higher in developed countries, like Japan and Korea, than in countries with low incomes in Asia. A Japanese study found that the prevalence of dementia equals 11% among those aged more than 65 years [35, 36], whereas a Korean one found the prevalence of 6.3% [17]. Another study conducted in Korea, the Seoul study, showed that the prevalence of dementia, excluding very mild cases, was about 5.3% for overall dementia and 4.3% for AD [37].

> The prevalence of dementia greatly varies between different ethnic groups living in the same country, like in Singapore, that is probably the most multicultural region of Asia. A Singaporean study showed low standardized dementia prevalence among the ethnic Chinese (2.5% among the elderly) when compared to the ethnic Malays (4.0% among the elderly) and this finding was independent of the frequency of vascular risk factors [18]. Whereas these differences are due to different genetics or lifestyle it remains a matter of debate.

However: it turns out that some of the usual case for yearly flu shots is little more hand-wavy than I'd realized.

Flu-like illnesses aren't as well-tracked as the stats might make you think - with an awful lot of grouping under "influenza-like illnesses" of non-flu diseases (that aren't helped by flu immunization), or tallying with all pneumonias. I have a strong sense that pre-COVID, normal "flu deaths" stats were already being exagerrated as part of the campaign to remind people to get flu shots.

For example, in late 2018, part of the CDC's push for more vaccinations estimated the previous flu season was the deadliest in memory, with 80k deaths, and that previously, the worst year had been 56k deaths: https://www.statnews.com/2018/09/26/cdc-us-flu-deaths-winter...

But in their current estimates, that same flu season only killed 52k - and not a single season of the last 10 reached 56k: https://www.cdc.gov/flu/about/burden/index.html

(Some have been pointing out such issues for a while, such as in this 2005 BMJ comment, "Are US flu death figures more PR than science?" – https://www.bmj.com/content/331/7529/1412.)

This actually came back to bite public health authorities during COVID when even the harder data on early COVID mortality could be characterized, against inflated flu-severity numbers, as "no more than 10x worse than a recent bad flu season". Truer (smaller) flu numbers likely would've led to instead an "at least 10x and probably 20x" comparison.

At the same time, it's quite hard to track the yearly effectiveness of flu shots against strains that often don't not match those chosen when vax production begins. In normal years, even suspected flu deaths don't necessarily lead to a lab-determined confirmation of any infectious agents involved.

You can see how little researchers trust the consistency of data on flu cases in the very research highlighted here. If you think flu vaxes prevent Alzheimer's, one obvious mechanism to theorize might be: actually getting the flu increases Alzheimer's risk, the shot reduces flu cases. But they don't even run that analysis, because they don't trust the data on actual flu cases suffered in their samples as being unbiased. (They write: "Fourth, we chose not to include influenza infections in our analyses because of concerns surrounding influenza infection misclassification in retrospective studies of administrative data [56]. Similar to the authors of a recent study investigating the flu vaccine’s association with dementia risk in a VA cohort [20], we concluded that the potential bias from frequent misclassification of infection status outweighed the potential benefits of including this factor in the analysis.")

Finally, and this is the kicker which might knock me off my previous habit of annual flu vaxes, until I'm at a more-frail age, is that there was growing recognition before COVID that regular, mismatched flu vaxes might actually increase susceptibility to some future strains.

You can see a bit of discussion of that in this 2019 guide to the "changing science of flu shots" – https://www.statnews.com/2019/10/14/updated-guide-changing-s... – specifically bottom "Can repeated vaccination actually backfire?" section.)

Two mechanisms that might contribute are imprinting (overspecializing immunity to the 1st strains seen, slowing later) and urgency-fading (training the immune system "we see a batch of these paper-tiger threats every year & they never create actual illness, so we can tamp down our responses to this whole class of agents").

This suggests to me that, even as a big fan of vaccination, I may want to use it somewhat more sparingly: when a specific strain has a higher risk, when a specific formula is well-tuned to an emerging threat. Otherwise, both for me & the community, I might be dissipating the benefits before they're urgent.