Risk of diphtheria among schoolchildren in the Russian Federation in relation to time since last vaccination

Background Between 1990 and 1996, more than 110 000 cases and 2900 deaths from diphtheria were reported in the Russian Federation. In 1994, because disease rates were high among children aged 7–10 years, the age of administration of the second booster dose of diphtheria vaccine was lowered from 9 years to 6 years, the age of school entry. To assess the impact of this policy change, we did a matched case-control study in three Russian cities. Methods Children aged 6–8 years who had diphtheria between September, 1994, and December, 1996, were each matched with five to seven children acting as controls who were within 3 months of age of the case and were from the same class at school. We did a matched analysis using conditional logistic regression. Findings We analysed the immunisation records of 58 cases and 306 controls. All but one case and all controls had received at least three doses of diphtheria-toxoid vaccine. 19 (33%) cases and 144 (47%) controls had received a booster dose of diphtheria toxoid within the previous 2 years. Cases were more likely than were controls to have received only four doses rather than five (odds ratio 2·8 [95% CI 1·2–6·5]) and to have a time since the last dose of diphtheria toxoid of 3–4 years (3·1 [1·1–9·1]) or 5–7 years (15·0 [2·5–89·0]), compared with children for whom it was 2 years or less. On multivariate analysis only a time since the last dose of 5–7 years remained significantly associated with disease (matched odds ratio adjusted for total number of doses 10·9 [1·6–75·1]). Conclusion A booster dose of diphtheria-toxoid vaccine given to children in the Russian Federation at 6–8 years of age reduced the interval since the last dose of diphtheria toxoid and improved protection against diphtheria.