Objective: Analyze the relationship between inoculating one case of the COVID-19 inactivated vaccine (Vero cell) and immune thrombocytopenic purpura to provide a reference for the standardized handling of adverse events following immunization. Methods: According to the “National Monitoring Program for Suspected Adverse Reactions to Vaccinations,” an on-site investigation, data collection and analysis, expert group diagnosis, and medical association assessment were conducted on a case of immune thrombocytopenic purpura in District A of Chongqing after vaccination with the inactivated COVID-19 vaccine. The assessment report was delivered to the three relevant parties, the case was reviewed, and the experience was summarized. Results: The investigation and diagnosis by the district-level vaccination abnormal reaction expert group concluded that the disease that occurred after vaccination with the COVID-19 inactivated vaccine was secondary immune thrombocytopenic purpura, an abnormal reaction to the vaccination. The medical damage was classified as Level II Grade B. The vaccine production enterprise raised objections to this conclusion. After re-assessment by the municipal-level medical association, the conclusion was consistent with that of the district-level medical association. The vaccine production enterprise did not raise any further objections. Conclusion: Through active collaboration among district and municipal-level medical associations, disease control institutions, and vaccination units, the recipients have been promptly and effectively treated, providing financial support for their subsequent treatment and safeguarding their rights. The investigation and disposal procedures for adverse events following immunization in Chongqing are clear, and the mechanism is sound. It is necessary to continue strengthening the monitoring of adverse events following immunization according to the existing plan and to ensure timely and standardized handling. Simultaneously, it is crucial to strengthen vaccine management and vaccination management.