“Without adjustment for other covariates, influenza vaccination was not protective in patients either with or without hypertension. After adjusting for comorbid

“Without adjustment for other covariates, influenza vaccination was not protective in patients either with or without hypertension. After adjusting for comorbidities, vaccination was strongly associated with a reduced risk of stroke in patients with hypertension; however, there was a non-intuitive interaction, contrasting with previous studies, such that vaccination in patients without hypertension was associated with a marginally higher risk of stroke compared with no vaccination. We observed that the non-hypertensive vaccinated population was much more likely to be female, and that the young average age of the non-hypertensive never-vaccinated stratum was associated with a small number of outcomes. A possible explanation for this finding is that the increase in stroke risk in vaccinated non-hypertensive individuals might represent vaccine-seeking behaviour due to other non-vascular risk factors for poor health outcomes, such as a history of cancer or autoimmune disease, conditions for which we could not adjust. In this situation, vaccination is acting as a latent variable, identifying these individuals at risk of stroke rather than being causative in increasing the risk of stroke. Another possibility is the misclassification of hypertension status by way of undiagnosed hypertension. We also do not have blood pressure measurement or medication data and therefore cannot classify the severity of hypertension or compliance with hypertension treatment, which might influence our findings. Alternatively, this finding might simply be due to chance.” Note that if you separate a population by enough factors, at least one will show an effect different from the aggregate, by pure probability.