The use of a vaccine against classical swine fever virus (CSFV) during an outbreak of CSF should lead to a reduction in the horizontal or vertical transmission of CSFV. The reduction of vertical, i.e. transplacental, transmission of a moderate‐virulent strain of CSFV from the sow to its offspring was studied in sows vaccinated once or twice with a CSFV E2subunit vaccine. Two groups of nine sows were vaccinated with one PD95dose of the E2subunit vaccine, approximately four weeks before insemination. A third group of nine inseminated sows served as controls. One group of nine sows were vaccinated again at two weeks after insemination. At ten weeks after the primary vaccination, approximately six weeks after insemination, all 27 sows were challenged intranasally with 105TCID50of a moderate‐virulent strain of CSFV, the Van Zoelen strain. The sows were euthanized at five weeks after challenge, and samples from the sows and fetuses were collected for detection of CSFV. All 27 sows were in gestation at the time of slaughter, CSFV was detected in the fetuses of all unvaccinated sows but it was not detected in any of the samples collected from fetuses of the double‐vaccinated sows. Virus was however recovered from the fetuses of one out of nine sows vaccinated once. All the sows, except four double‐vaccinated sows, developed CSFV Ernsantibodies. Transplacental transmission of CSFV was reduced significantly (p <0.001) in all vaccinated sows. When the results from the experiment were extrapolated to a herd level, it could be concluded that, with 95% certainty, approximately 11% (single vaccination) or 0% (double vaccination), confidence intervals of 0.01–0.44 and 0.0–0.30 respectively, of the pregnant sows would still not be protected against vertical transmission of moderate‐virulent CSFV.