Immunity to diphtheria and tetanus in Australia: a national serosurvey

Objective: To determine immunity to tetanus and diphtheria in the Australian population. Design and setting: Analysis, using double antigen enzyme immunoassays, of a representative sample of sera (1950 samples tested for diphtheria and 2884 for tetanus) collected opportunistically from Australian laboratories between July 1996 and May 1999. Main outcome measure: Immunity to diphtheria and tetanus, defined as negative (susceptible) when the antitoxin level was < 0.01 IU/mL, positive (immune) when it was >= 0.1 IU/mL, and low positive (partially immune) when it was in the range 0.01-0.1 IU/mL. Results: About 99% of children aged 5-9 years had diphtheria and tetanus antitoxin levels >= 0.01 UJ/mL (immune or partially immune). Antitoxin levels declined with age and generally more markedly for diphtheria than tetanus. For subjects aged 50 years and over, less than 60% were immune or partially immune to diphtheria and less than 75% to tetanus. Men and women had similar diphtheria antitoxin levels, while women had lower levels of tetanus antitoxin compared with men of the same age, with the difference being most marked in the age group >= 70 years (37% v 60%; P < 0.001). Conclusions: Immunity in children appears to be good, but adults, especially older people, may not be adequately protected. Recent changes to the Australian Standard Vaccination Schedule should improve immunity in cohorts now aged < 50 years. However, additional efforts are required to protect those over 50 years (especially travellers), who are most susceptible.