This vaccine is used to keep up protection (immunity) against diphtheria, tetanus (lockjaw) and pertussis (whooping cough) in children and adults who have been vaccinated for these diseases in the past. It may also be given during the third trimester of pregnancy to help prevent pertussis in the newborn baby. Vaccination is the best way to protect against these life-threatening diseases. Vaccines work by causing the body to produce its own protection (antibodies). Booster doses are needed to keep up immunity because antibody levels may become too low over time to provide the needed protection.