Abstract We present a systematic determination of the order ℏ fermionic energy shift when an electroweak string is perturbed. We show that the combined effect of zero-modes, bound states and continuum states is to lower the total fermionic ground state energy of the string when the Higgs instability of the string is excited. The effect of the Dirac sea is thus to destabilise the string. However, this effect can be offset by populating positive energy states. Fermions enhance the stability of an electroweak string with sufficiently populated fermionic bound states.