In a recent question on superpositions of different quarks it was explained, that the superpositions of different electric charged particles cannot exist, in contrast to strangeness quantum number. It was reasoned that this is because of decoherence that comes from the QED vacuum - the photon interacts ("measures") with charged particles -> thus their charge is known -> destroys decoherence.
But of course, double slit experiments with single electrons have been performed already in the 70s.
Why do virtual photons not cause decoherence of the spatial coherence of single electrons?
If they would cause decoherence, it would result in reduction of the contrast in single electron double-slit experiments.