  In the standard model, stabilization of a classically unstable cosmic string
may occur through the quantum fluctuations of a heavy fermion doublet. We
review numerical results from a semiclassical expansion in a reduced version of
the standard model. In this expansion the leading quantum corrections emerge at
one loop level for many internal degrees of freedom. The resulting vacuum
polarization energy and the binding energy of occupied fermion energy levels
are of the same order, and must therefore be treated on equal footing.
Populating these bound states lowers the total energy compared to the same
number of free fermions. Charged strings are already stabilized for a fermion
mass only somewhat larger than the top quark mass. Though obtained in a reduced
version these results suggest that neither extraordinarily large fermion masses
nor unrealistic couplings are required to bind a cosmic string in the standard
model. Furthermore we also review results for a quantum stabilization mechanism
that prevents closed Nielsen-Olesen type strings from collapsing.
