Re-emergence of diphtheria in Kerala: the need for change in vaccination policy

Background Diphtheria cases continue to occur in India despite a national vaccination program targeting the disease. Outbreaks of diphtheria have been known to occur in areas of low immunization coverage. An age shift has been noted to older children and adults in recent outbreaks from the Indian states of Andhra Pradesh, Karnataka, Delhi and Assam. Kerala witnessed its largest outbreak of Diphtheria in recent times from 2015 to 2017.Methods: Surveillance data from the Regional PEID Cell during the outbreak period was analysed and epidemiological data generated.Results: A total of 734 cases of diphtheria were reported during this period with eight deaths (Case fatality rate=1.08%). The mean age of the cases was 17.4 years (±13.9). More than 72% of the cases occurred in children above 10 year of age and 68% of the cases were either unimmunized or partially immunized. 32% of the cases were immunized for age. Of these, immunized children, 88% were above 10 years of age, indicating waning immunity with age. The existence of a sizeable unimmunized cohort in the adolescent age group and waning immunity among immunized were two major factors contributing to the outbreak.Conclusions: An age shift has been observed in the occurrence of Diphtheria cases during the outbreak in Kerala. Booster doses with Td vaccine during adolescence in addition to maintaining a high immunization coverage in the routine immunization program, with special emphasis on pockets of low coverage is essential for preventing the reemergence of diphtheria.