  We investigate the effects of matter-enhanced (MSW) transformation of
neutrinos in the Early Universe on the primordial helium yield (Y_p). We find
that Y_p is affected much more by the MSW-induced alterations in the neutrino
energy spectra than by the associated change in expansion rate. Specifically,
the absence due to transformation of low energy electron neutrinos can
significantly affect neutron-proton weak interconversion rates through the
lifting of Fermi-blocking of neutron decay at low energies and through halting
low-energy neutrino capture on neutrons. We find that the change of Y_p within
a causal horizon is -0.005\le \delta Y_p \leq 0.013 for m_{\nu_{\mu,\tau}}^2 -
m_{\nu_s}^2 \le 10^4 eV^2 in the case of \nu_{\mu,\tau}-\nu_s-\nu_e mixing,
with the lower limit at m_{\nu_{\mu,\tau}}^2 - m_{\nu_s}^2 \approx 100 eV^2,
and -0.002\le \delta Y_p \leq 0.020 for m_{\nu_e}^2 - m_{\nu_s}^2 \le 1 eV^2 in
the case of \nu_e-\nu_s mixing.
