[omniglottal]

Why not plural microorganismS? Although a neglible increase of mortality across countless events where broad-acting immunity protects us may be outside your ability to calculate, it's still readily applicable to recognize that a lot of straw exists on a camel's back without simply studying just the one which broke it. An overall increase in mortality from all causes is congruent with valid scientific theory and is a hypothesis worth evaluating without eschewing existing data. Your question is unneccessarily leading with false premises by which you define the answer without accepting the known truth. If you're looking for just one organism which would need to spread, then you're looking in the wrong place for how, i.e., people were killed by myocarditis, or inability to stay nourished while suffering vaccine-induced side effects, including inability to defend with the same metabolic fervor from the flu.