What studies exist for past flu seasons’ vaccine effectiveness?

The short version: the evidence retrieved contains no studies at all, so there is nothing to evaluate for past flu seasons either.

The evidence block you provided is empty. No trials, no surveillance data, no manufacturer-funded papers, no independent analyses of any past season's vaccine effectiveness. Without a single study to examine, I cannot tell you the effectiveness by season, subtype, or population. That gap is itself the finding.

The question asks for a specific evaluation based on past seasons' data. The answer is that the evidence retrieved does not address it. Any number I gave you would be a guess drawn from memory, which is fabrication. The honest answer is that the evidence is absent.

My call: the retrieved evidence does not answer the question. Confidence: not applicable — there is no evidence to weigh.