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.

•Two-thirds of adults had antibody below the protective level during a diphtheria outbreak.•After one dose of Td, 87% achieved seroprotection against diphtheria.•Lowest seroprotection was reported among adults aged 30–49 years.•One dose of Td is recommended, and a 2-dose series is considered for adults born before 1980.