A study of the sero-immunity that has protected the Swedish population against poliomyelitis for 25 years

Although the population was highly susceptible to poliomyelitis on account of its low natural immunity, the general vaccination with inactivated polio vaccine eliminated poliomyelitis in Sweden within 6 years (1957-62). This status remains after 25 years, although challenges from the introduction of virus occur constantly. Over 99% of the population born in the forties and later is estimated to have been vaccinated, as well as at least 90% of those born earlier. The mean antibody levels of vaccinees against polio range from 100 (type 3) and 500 (type 1) up to 800 (type 2). Seronegative persons are seen only in a small percentage. Evaluation of the duration of immunity indicates that, after the post-vaccination fall in titre which occurs within 2-5 years after immunization, the immune levels remain fairly stable or decline very slowly. Sera from 12-year-old Swedish vaccinees neutralized the type 3 Saukett strain used for vaccine production 5 times better than a type 3 strain isolated during the Finnish outbreak 1984.