The annual flu shot is a guess. Due to manufacturing lead time and distribution the prediction for which strain will be the dominant strain in the future is made (afaik) approx a year earlier.

The point being, its possible any give year's jab is not the ideal jab given the strain that ended up the actual dominant.

Point being, the study should account for that, or at least explain the significance (or insignificance) of the match / mismatch. That is, is the jab creating an immune response that mitigates stroke. Or is the jab creating an immune response that mitigates flu and less flu means less stroke.

Otherwise, this feels like correlation.