  Correction to the hyperfine constant $A$ of an external s-electron in
many-electron atom caused by the Breit interaction is calculated analytically:
$\delta A/A =0.68 Z\alpha^2$. Physical mechanism for this correction is
polarization of the internal electronic shells (mainly $1s^2$ shell) by the
magnetic field of the external electron. This mechanism is similar to the
polarization of vacuum considered by Karplus and Klein long time ago. The
similarity is the reason why in both cases (Dirac sea polarization and internal
atomic shells polarization) the corrections have the same dependence on the
nuclear charge and fine structure constant.
  In conclusion we also discuss $Z\alpha^2$ corrections to the parity violation
effects in atoms.
