An open clinical trial to study the immune response and safety after giving a booster dose (5th Dose) of a combination vaccine against Diphteria-Tetanus-Pertussis-Polio to healthy adolescents 15-16 Years of age. The first three doses were given during the first year of life, according to the Norwegian child immunization program. The fourth dose was given in a previous clinical trial performed in 1998 when the children were 6-7 years old.