Susceptibility to Diphtheria among Vaccinated Children Aged 1–14 Years: A Serosurvey in a Rural Subdistrict, Bangladesh, 2019

In Bangladesh, diphtheria vaccine is administered to children at six, ten and fourteen weeks of age, with no booster dose given up to 14 years. As antibody levels decrease over time, over five years may become susceptible to diphtheria. We conducted a serosurvey in a rural subdistrict among children aged 1–14 years to estimate the proportion of protected children, to determine the mean anti-diphtheria antibody concentration, and to identify factors associated with being unprotected in vaccinated children. We enrolled children who had completed their primary diphtheria vaccination and selected three age groups: 1–4 (n=422), 5–9 (n=518), and 10–14 years (n=402), using cluster sampling across in a selected rural subdistrict. We collected information on socio-demographic, environmental, maternal and birth-related characteristics, and vaccination history. Serum diphtheria antibody levels were measured by ELISA, with ≥0.1 IU/mL considered protective. We used chi-squared test and odds ratio to identify associated factors and applied multiple logistic regression to adjust for confounding. The proportion of protected children in the three age groups was 76% (1–4), 48% (5–9) and 36% (10–14). The geometric mean titer (GMT) of diphtheria antibody was highest in 1–4 years (0.18 IU/mL) and lowest 10–14 years (0.08 IU/mL). Being in older age groups were significantly associated with being unprotected. Over half of children aged 5–9 years and two-thirds of children aged 10–14 years were unprotected. We recommend introducing a booster dose of diphtheria vaccine for children over five years.