All the member States of the EU are affected by the Covid-19 pandemic. Various measures to tackle the health crisis have been (or could be) taken at different levels: e.g. the possibility to emit "Coronabonds" at the EU level, lockdown measures at the national level (e.g. France, Italy, but not Sweden in March/April 2020), and different restrictions at the level of cities (e.g. mask wearing, additional curfew measures).

What is the principle determining which is the adequate "level" to take such decisions?