 While the immune mechanisms involved are the same, the level of immune protection required for foetal protection is much higher than for prevention of clinical disease.While challenge studies indicate that killed, as well as live, vaccines prevent foetal infection under experimental conditions, the efficacy of vaccines under field conditions has been questioned.The birth of PI calves into vaccinated herds suggests that killed vaccines do not stand up to the challenge presented by the viral load excreted by a PI in the field.