Persistent high diphtheria positivity in North India: Evidence from a tertiary hospital-based study, 2007-2021

Background and objectives Despite the availability of effective vaccines, diphtheria continues to cause morbidity and mortality in parts of India. We analysed 15 yr of hospital-based data from a tertiary care infectious disease hospital to describe the epidemiology of diphtheria in northern India. Methods We analysed data from January 2007 to December 2021 on suspected diphtheria cases (defined as upper respiratory tract illness with an adherent membrane in pharynx, tonsils, larynx, or nose) hospitalised at an infectious diseases hospital. Information on demographic characteristics, vaccination status, laboratory findings, and clinical outcomes was abstracted from the records. We described the distribution of diphtheria by person, place, and time. Multivariable logistic regression was used to examine the association between vaccination status and culture-confirmed diphtheria. Results Among 5,493 suspected diphtheria cases hospitalised during the study period, C. diphtheriae was isolated from 2,131 (38.8%). 408 diphtheria patients died (case-fatality ratio: 19.1%). Culture positivity among suspected cases remained consistently high (range: 15% to 56%) over time. The median age of culture-confirmed cases increased from 5 yr [interquartile range (IQR) 4-8)] during 2007-2011 to 7 yr (IQR 5-9) during 2012-2016 and 7 yr (IQR 6-10) during 2017-2021. Most (n=1876, 88.0%) cases were from Uttar Pradesh and Delhi. Only 23 (1.1%) reported being fully vaccinated for age. Compared with fully vaccinated individuals, partially vaccinated [adjusted odds ratio (aOR): 4.5; 95% confidence interval (CI): 2.8-7.4] and unvaccinated or unknown-status individuals (aOR: 4.2; 95% CI: 2.7-6.5) had higher odds of having diphtheria. Interpretation and conclusions Sustained high diphtheria positivity over 15 yr indicates ongoing transmission in parts of northern India, likely driven by immunity gaps due to suboptimal booster-dose coverage. Strengthening the Universal Immunization Programme with focus on timely booster doses among preschool- and school-aged children is essential to reduce transmission.