DPT for the fourth and fifth doses. Diphtheria and tetanus toxoids for pediatric use (DT) should be used in children younger than age 7 years in whom DPT is contraindicated. Tetanus and diphtheria toxoids for adult use (Td) should be used in individuals older than age 7 years. They should be administered every 10 years following the last DPT or DT vaccination.

BIOLOGIC EVENTS FOLLOWING IMMUNIZATION

Tetanus

Following an injection of tetanus toxoid, the recipient develops neutralizing antibodies that prevent the effects of toxin on the nervous system. Antibody levels are now reported in comparison with an international standard set by the WHO as international units per milliliter, and it is generally agreed that a level of 0.01 IU/ml or greater is protective (Wassilak and Orenstein, 1988). Protective levels of antibody are achieved in most children and adults after two doses of tetanus toxoid given 4 or more weeks apart, although children under 1 year of age may require three doses of tetanus toxoid (Barkin et al., 1984, 1985a). However, protective levels are relatively short-lived, particularly in infants and older adults, and thus, a reinforcing (booster) dose is given 6 to 12 months after the primary series of immunizations. Following this booster, long-term immunity usually exceeding 10 years develops (Peebles et al., 1969).

Minor local reactions (pain, erythema, swelling of less than 1 cm) occur within 48 hours following 1-80 percent of immunizations with tetanus toxoid (Collier et al., 1979; Jones et al., 1985; White, 1980). The reaction rate varies with the dose and type of toxoid, the number of prior doses of toxoid received, and the method of injection. Severe reactions (>8 cm of erythema or induration) are much less common and are often accompanied by a sore, swollen arm and systemic manifestations such as fever and malaise. Severe reactions occur much more frequently with larger doses of toxoid (McComb and Levine, 1961; Schneider, 1964), and in several studies, severe reactions have been found to correlate with high antibody levels prior to immunization (Collier et al., 1979; Facktor et al., 1973; Korger et al., 1986; Levine and Edsall, 1981; Levine et al., 1961; McComb and Levine, 1961; Relihan, 1969; White et al., 1973). Prior to the recognition of the long duration of immunity, frequent booster doses given after minor wounds or as prophylaxis for factory employees or children attending summer camps led to high levels of antibody.

The correlation between severe local reactions and high antibody levels prior to immunization strongly suggests that these reactions may be caused by immune complex formation between antibodies and antigen. In the case of severe local reactions, this is classified as an Arthus reaction, in which



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