This is just confirmed the draft entered into law, which is as expected because they earlier stated that they will do this while they wait on regulatory agencies to decide on a forth dose.
In attesa che le agenzie regolatorie si pronuncino sulla eventuale quarta dose, il governo ha dunque deciso di garantire la «copertura» in modo che non ci sia il rischio che la certificazione perda validitą.
Obviously a dose has "indefinite validity" if the vaccination programme ends because it's the last jab.