  We investigate the Hadamard function, the vacuum expectation values of the
field square and the energy-momentum tensor of a scalar field with general
curvature coupling parameter in de Sitter spacetime compactified along one of
spatial dimensions. By using the Abel-Plana summation formula, we have
explicitly extracted from the vacuum expectation values the part due to the
compactness of the spatial dimension. The topological part in the vacuum
energy-momentum tensor violates the local de Sitter symmetry and dominates in
the early stages of the cosmological evolution. At late times the corresponding
vacuum stresses are isotropic and the topological part corresponds to an
effective gravitational source with barotropic equation of state.
