Background: Despite the unquestionable public health benefits of immunization, rare incidents of Adverse Events Following Immunization (AEFI) remain a cause of concern. Following few deaths after vaccination in a village of Jharkhand, India, an epidemiological investigation was carried out by the experts of a medical college to find out the cause of deaths and recommend state health further to prevent such incidences in future. Materials and Methods: The epidemiological investigation was conducted as a field study which comprised of observational visit of the AEFI site, key interviews of all stakeholders, verbal autopsy and desk review of all related documents to immunization. All relevant laboratory and autopsy investigations were done to exclude or include all possible causes. Results: On investigation, it was noted that there were five children who developed symptoms of severe gastro-enteritis within six hours of vaccination out of the total thirteen vaccines from the single immunization site and four of them died within 24 hr of vaccination suggestive of features of Toxic shock syndrome. All had a common exposure of Measles vaccine and Vitamin A giving hint towards contamination of vaccine during reconstitution or administration of vaccine. In autopsy findings, there was no external injury reported and the laboratory findings of vaccine vial didn’t reveal any significant findings. The attack rate of the event was 100% and case fatality rate was 80%. Conclusion: From investigation, it was concluded that the events are due to Immunization error related reaction (a type of AEFI) and causality association tool also classified the event to be consistent causal association to immunization. There is dire need of refresher training for service providers on various aspects of AEFI to avoid recurrences of such events.