The COVID-19 vaccines provide a high degree of protection against severe disease, hospitalisation, and death. However, no vaccine claimed 100% effectiveness and it is expected that a small proportion of vaccinated individuals may develop a breakthrough infection due to individual differences, virus variants and other factors. We conducted an epidemiological investigation and analysis of an imported case who had finished four doses of vaccination, and in order to provide a relevant reference for regular epidemic prevention and control in the post-pandemic era.