Every tetanus shot I've had involved a very intense sporting or outdoor injury that needed stitches. The cohort getting these are vastly more active than those who don't, in a way that's far more accurate than self reported "exercise."
Basically everything good is correlated with lower dementia risk; it doesn't take half a brain for any of the authors to have realized this.
Comorbid physical and psychiatric conditions were defined by ICD-9 and ICD-10 codes and included type 2 diabetes, obesity, hypertension, stroke, ischemic heart disease, congestive heart failure, atrial fibrillation, asthma, chronic obstructive pulmonary disease, traumatic brain injury, vitamin B12 deficiency, depression, anxiety disorder, nicotine dependence and alcohol or drug abuse/dependence.
This would significantly reduce that problem.
The authors don't think the correlation is specifically the tetanus shot either, per the conclusion in the single page abstract. This paper's abstract (not an article) is just clearly and unambiguously stating that they have found a very strong correlation. The authors suspect (but don't know) that the tetanus shot is reducing inflammation, along with several other vaccines which are believed to have similar correlations but appear to have not yet been studied. Other studies have already linked inflammation with dementia, such as the ones showing patients can have the physical manifestation of Alzheimer's disease but not exhibit the symptoms of dementia, so this line of thought is very interesting and will hopefully lead to further studies. Perhaps the right anti-inflammatory medication could act as a prophylactic against dementia or even be able to treat dementia. Perhaps it will even point to brain inflammation as the reason that 'everything good is correlated with lower dementia risk'.