A single booster dose of diphtheria vaccine is effective for travelers regardless of time interval since previous doses.

Our study showed the immune response before and after a booster against diphtheria given within the 20-year interval recommended in Sweden or after a prolonged interval. Of 40 travellers, 10/13 in recommended interval group were immune before booster and 19/27 with a delayed interval. After booster, 13/13 versus 26/27 were protected. One booster was sufficient to achieve immunity regardless of the interval.