The placebo has to be something that feels like a vaccine, otherwise the double-blind aspect will be broken. But since we know the precise risks associated with

The placebo has to be something that feels like a vaccine, otherwise the double-blind aspect will be broken. But since we know the precise risks associated with meningitis vaccines, we can trivially determine the relative risk of the covid vaccine compared to something else of a known risk level. Oh, and it's not like the meningitis vaccine will confer any protection from covid.