  The vacuum-polarization corrections to rates for nuclear-fusion reactions in
the $pp$ chain and in the CNO cycle are calculated. For the reactions of
particular importance to the solar-neutrino problem, the $\Hethree$, $\Hefour$,
$\Be$, and $\Ni$ reactions, we find the magnitude of the effect to be less than
2\%. The effect of VP on all the other reaction rates is expected to be of a
similar order of magnitude. We discuss how these results affect the predicted
fluxes of solar neutrinos.
