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Interim vulgato Agrippinae periculo, quasi casu evenisset, ut quisque...

Note that "quasi" does not here introduce an unreal condition; it introduces virtual o.o., with the meaning "in the belief that," "on the ground that," a Taciten usage. So also with "tamquam." Here the meaning is that reports had circulated among the people ("vulgato") that the danger to Agrippina had occurred "by chance" ("casu").