Influenza is an acute respiratory infectious disease caused by influenza virus that seriously endangers people’s health. Influenza vaccination is the most effective means to prevent influenza virus infection and its serious complications. MDCK cells are considered to be superior to chicken embryos for the production of influenza vaccines, but the tumorigenicity of cells is a concern over the theoretical possibility of the risk of adverse events. The theoretical risks need to be adequately addressed if public confidence in programs of immunization are to be maintained. In this paper, studies of the tumorigenic potential of cell lines, with MDCK cells as an example, published since 2010 are reviewed. The mechanism of tumorigenicity of MDCK cells is discussed with reference to cell heterogeneity and epithelial to mesenchymal transition (EMT). Understanding the mechanism of the acquisition of a tumorigenic phenotype by MDCK cells might assist in estimating potential risks associated with using tumorigenic cell substrates for vaccine production.Copyright © 2023 International Alliance for Biological Standardization. Published by Elsevier Ltd. All rights reserved.