Seroprotection rate in adults after 1-dose, 2-dose, and 3-dose series of a reduced-diphtheria-tetanus toxoid vaccine (Td) during a diphtheria outbreak in Thailand.

To evaluate seroprotection of different dosing strategies of reduced-diphtheria-tetanus-toxoid vaccine (Td) for adults during a diphtheria outbreak in Thailand, we enrolled 160 healthcare workers and 161 adults aged 20-60 years old and measured diphtheria antitoxin (DAT) level before administration of a Td vaccine. We scheduled a second Td at 4-8 weeks and a third Td at 6-12 months interval. DAT was measured 4 weeks after each dose. DAT levels of ≥0.1 and ≥1 IU/mL were considered as seroprotective and long-term seroprotective. Persons achieving long-term seroprotection were not given a further dose. The baseline seroprotection rate was 32.6%, which increased to 87.1% (95% confidence interval, 83.4-90.8%) after one dose. The seroprotection rate increased slightly with additional doses. The immune response was lowest among persons 30-49 years of age. We suggest 1-dose Td for adults during a diphtheria outbreak, and a 2-dose series being considered for those born before 1980.