[Duration of tetanus immunity in relation to the number of doses of vaccine].

A study of the duration of the immunity to tetanus, by means of the passive haemagglutination test, was undertaken in three groups of subjects immunized with two (primary immunization), three (basic immunization) and four (basic immunization with booster) does of DT. All the subjects of the first group, who had gone 9 months since their last dose, possessed antitoxin titers greater than or equal to 0.01 I.U./ml. Among the subjects of the second group within 12 months of the basic immunization course none had less than 0.01 I.U./ml, within 13-48 months 3% had less than 0.01 I.U./ml, within 49-72 months 9.7% had less than 0.01 I.U./ml and after 73 months 26.5% had less than 0.01 I.U./ml. No dependence on age at time of vaccination could be demonstrated. In the third group of subjects within 48 months of the immunization course with four doses none had less than 0.01 I.U./ml, after 48 months 11.1% had less than 0.01 I.U./ml. Interpolation of the data with the use of linear regression (y = a + bx), which better-fits in comparison with polynomial model (y = a + b1x + b2x2...+ bnxn) until the quartic term, pointed out the possibility that titer falls below the "alert level" (0.1 I.U./ml) and the protective threshold (0.01 I.U./ml): after 22 and 34 months for subjects of the first group; after 110 and 180 months for subjects of the second group and after 164 and 269 months for subjects of the third group. However, the presence of "poor" or "not responders" in the second group is not negligible and justifies the necessity that the first booster dose be given at five-year intervals.