How many doses of diphtheria toroid are required for protection in adults?Results of a case-control study among 40-to 49-year-old adults in the Russian Federation

During the Russian diphtheria epidemic of the 1990s, adults had an unexpect edly high rate of disease. A retrospective, matched case-control study was done to measure the effectiveness of one, two, or three or more doses of di phtheria toroid against diphtheria among 40- to 49-year-old Russians. Thirt y-nine diphtheria case-patients and 117 controls were studied. Previous vac cinations were included if one dose was received within the previous 10 yea rs. Five cases (13%) and 33 controls (28%) had received three or more doses of vaccine. The matched odds ratio was 0.3 (95% confidence interval, 0.1-0 .9) for three or more doses compared with no doses, which was a vaccine eff ectiveness of 70% (95% confidence interval, 10-90), A trend existed toward milder disease with increasing doses (chi(2) test for trend, P = .02), The results suggest that Russian adults, who were unlikely to have acquired imm unity to diphtheria through immunization or natural infection, required at least three doses of diphtheria toroid for reliable protection against dise ase.