Abstract
Background amidst a global resurgence of diphtheria cases with numerous outbreaks recorded worldwide since 2000, better understanding of this vaccine-preventable disease’s circulation is needed. Methods we analyzed retrospectively sera from two sero-epidemiological cross-sectional studies in Madagascar and Cambodia on fully primo-vaccinated three to fifteen year olds. Using ELISA and seroneutralization (Vero Cell TNT) for the three to eight year olds with low ELISA titration values (<0.01 IU/mL), we assessed the (i) duration of protection by primary vaccination for three to eight year olds, and (ii) 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 more than 6 years post vaccination. Results 745 children in Cambodia and 949 children in Madagascar were included. Our results show significantly more unprotected children among the five/six year olds than among the three/four year olds, with 41.1% (39/95) vs. 26.7% (27/101) (p=0.03) in Cambodia and 21.4% (27/126) vs. 8.0 % (9/113) (p<0.01) in Madagascar. In Cambodia and Madagascar, respectively 27.8% and 20.7% of the participants whose primary vaccination was performed more than 6 years earlier were seropositive, 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 vs. 5–6 years earlier. Conclusion 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 six years of age and for adolescents, as recommended by WHO.