Adverse event reporting rates following tetanus-diphtheria and tetanus toxoid vaccinations: data from the Vaccine Adverse Event Reporting System (VAERS), 1991-1997.

Since 1966, the Advisory Committee on Immunization Practices (ACIP) has recommended tetanus-diphtheria toxoid (Td) be used instead of single antigen tetanus toxoid (TT) because, while both vaccines protect against tetanus, only Td protects against diphtheria. Despite this recommendation, approximately 2.5 million doses of TT were distributed annually from 1991 to 1997. One possible explanation for the continued use of TT is concern about the relative safety of Td. Small clinical trials found Td to be associated with a higher rate of local vaccine-associated adverse events (VAEs) than TT. To determine if the findings from the trials would hold up on a larger scale, we compared the rate of reporting to the Vaccine Adverse Event Reporting System (VAERS), a passive reporting system, after either vaccine from 1991 to 1997. There were 40 reports per million doses of Td, and 27 reports per million doses of TT, for a reporting rate ratio of 1.4. Reporting rates to VAERS are lower than the rates of VAEs identified in the clinical trials, but the magnitude of the difference in VAEs following TT versus Td is similar. While reporting rates are lower after TT than Td, rates of reported VAEs after both vaccines are low.