Seroprevalence of Diphtheria in Antananarivo, Madagascar, and Cambodia

Background. Amidst a global resurgence of diphtheria cases with numerous outbreaks recorded worldwide since 2000, a better understanding of this vaccine-preventable disease’s circulation is needed. Methods. We retrospectively analyzed sera from 2 sero-epidemiological cross-sectional studies in Madagascar and Cambodia on fully primo-vaccinated 3- to 15-year-olds. Using enzyme-linked immunosorbent assay (ELISA) and seroneutralization (Vero Cell TNT) for the 3- to 8-year-olds with low ELISA titration values (< 0.01 IU/mL), we assessed (i) the duration of protection byprimary vaccination for 3- to 8-year-olds and (ii) the level of diphtheria in children and adolescents. Seropositivity was defined as a titration value (by ELISA or TNT) of at least 0.1 IU/mL and was used as a proxy for diphtheria infection among individuals > 6 years postvaccination.Results. Seven hundred forty-five children in Cambodia and 949 children in Madagascar were included. Our results show significantly more unprotected children among the 5- to 6-year-olds than among the 3- to 4-year-olds, with 41.1% (39/95) vs 26.7% (27/101; P = .03) in Cambodia and 21.4% (27/126) vs 8.0% (9/113; P < .01) in Madagascar. In Cambodia andMadagascar, respectively, 27.8% and 20.7% of the participants whose primary vaccination was performed >6 years earlier wereseropositive, suggesting diphtheria infection. In both countries, we observed a higher rate of infected children when the last vaccine injection had been received 7–8 years or 11–12 years earlier vs 5–6 years earlier. Conclusions. Our data show that the disease is present at high levels in Cambodia and Madagascar and that the national recommendation—primary vaccination—is not sufficient: Booster doses appear necessary at around 6 years of age and for adolescents, as recommended by the World Health Organization.