Protective antibody IgG titres for diphtheria and pertussis among prison inmates in North India

Abstract Background Prisons being closed spaces are at high risk of respiratory infections. With the increasing number of outbreaks of diphtheria and pertussis among adults, it is imperative to know the level of protective antibody titres among prison inmates to prevent such outbreaks. Methods India conducts HIV Sentinel Surveillance (HSS) in central prisons every two years to estimate HIV prevalence among inmates. The leftover serum samples of the HSS 2023 round were used to test for diphtheria and pertussis toxin IgG antibody titres using ELISA kits from Serion. From 6 prisons in North India, 1247 randomly collected samples of inmates were tested. The titres for diphtheria were interpreted as < 0.1 IU/mL: Insufficient protection (requiring immediate booster vaccination); 0.1 IU/mL to 1.0 IU/ml: Minimal protection (requiring booster for long term protection); and >1.0 IU/mL: Safe protection (requiring booster after 5-10 years). The antibody titres for pertussis were interpreted as < 40 IU/mL as negative (susceptible to infection); 40 to 100 IU/mL as borderline; and >100 IU/mL as positive (had recent infection or vaccination) and were currently immune. Results Among prison inmates, 25% had safe protection, 72% had minimal protection whereas 3% had insufficient protection against diphtheria. Bordetella pertussis IgG antibody was positive among 14.5% indicating recent infection as last vaccination dose is given at 5 years of age under National Immunization Schedule, borderline in 19%, and negative in 66.5% of the inmates. Conclusions Protective titres wane with time. Diphtheria booster may be recommended every 10 years for adults as most of them had minimal protection. One-sixths had evidence of recent infection implying that pertussis is underdiagnosed. Prisons have high risk of outbreaks as two-thirds of inmates were susceptible to infection. Key messages • Protective immunity wanes with time and adult booster doses may be considered for diphtheria every ten years. • Pertussis among adults is underdiagnosed and large proportions were susceptible. This mandates further studies on epidemiology and cost-effectiveness of pertussis vaccines.