Chlamydiosis is only one of the causes of abortion in sheep. If the abortion rate remains unchanged in flocks which have been vaccinated with CEVAC® Chlamydia, it is recommended that veterinary advice is sought. The epidemiology of chlamydial abortion in ewes involves a long incubation period. Ewes that abort in any lambing season have usually been infected at the previous lambing. Field trial data indicate that vaccinating incubating ewes will reduce the incidence of abortion, but a proportion can still go on to abort. Susceptible in-contact vaccinated ewes are protected. A good immune response is reliant on the reaction of an immunogenic agent and a fully competent immune system. Immunogenicity of the vaccine antigen will be reduced by poor storage or inappropriate administration. Immunocompetence of the animal may be compromised by a variety of factors including poor health, nutritional status, genetic factors, concurrent drug therapy and stress.