Summary: More than 100 persons died within 7 days of infl uenza vaccination by November 2020 (for two months of 2020 fl u vaccination) in the Republic of Korea (South Korea). The current study was conducted to allocate properly any possible causality by examining the presence of heavy metals in a vaccine and metal-induced immunotoxic lesions after fl u vaccinations in the experimental mice. It detected cadmium 0.12 ppb (parts per billion = μg/L) and mercury 1.77 ppb in one of the cost-free infl uenza vaccinees (Lot Number: A14720017) distributed by the Korean government. Lungs of the undiluted-vaccine-injected mice showed signifi cantly more diff use infl ammatory damages than lungs of the 1:4 dilutedvaccine-injected mice which showed no to mild infl ammatory changes (p < 0.027 by the method 1, and p < 0.001 by the method 2). Based on this study, it can be presumed that the metals-induced immunotoxicity of type IV hypersensitivity or of psuedoallergy would have caused death in some of persons who coincidentally died within 7 days of vaccinations. Background : A 17-year-old man died within three days of infl uenza vaccination (Lot Number: A14720007), a 77-year-old woman died within a day (Lot Number: A14720016), and more than 100 persons died within 7 days of infl uenza vaccination by November 2020 (for two months of 2020 fl u vaccination) in the Republic of Korea (South Korea). Singapore authorities halted two of the seven brands of fl u vaccinees even though there was no report of death after fl u vaccinations in Singapore. This raises a possibility that there can be a diff erence between the excipients of the fl u vaccinees used in Singapore and in Korea. Our assumptions were that there would have been immunotoxic metals in the fl u vaccinees, the metals would have induced type IV hypersensitivity or Complement Activation-Related Psuedoallergy (CARPA), and would have caused some deaths of 100 persons who incidentally died within 7 days after fl u vaccinations. Methods: In this study, we analyzed twice for the presence of any metal components of aluminum, indium, cadmium, gallium, and mercury in the infl uenza vaccine. Analysis of the metal contents of the 1:29 diluted fl u vaccine was assessed by the Inductively Coupled Plasma Mass Spectrometry (ICP-MS) method. Simultaneously, total 10 BALB/c mice were used to analyze any pathological changes after 7 days of fl u vaccination. Animals were divided into two groups: one group of 5 mice were injected intraperitoneally with 0.1 ml of 1:4 diluted fl u vaccine with injectable distilled water; and the other group of 5 mice were injected intraperitoneally with 0.1 ml of undiluted fl u vaccine. They were freely reared for 7 days in a Polycarbonate cage (400 x 255 x 180 mm). The mice were sacrifi ced after CO2 short-acting gas anesthesia. Brains, hearts, lungs, livers, and kidneys were harvested, prepared with H & E stain, and observed for any histopathological changes. Findings : In one of the cost-free infl uenza vaccinees (Lot Number: A14720017), which were distributed by the Korean Government, the current study detected cadmium 0.12 ppb (parts per billion = μg/L), and mercury 1.77 ppb. But neither aluminum, gallium, nor indium was detected. Both experimental groups showed no demonstrable infl ammatory changes in the specimens of brains, hearts, livers, and kidneys. However, lungs of the undiluted-vaccine-injected group showed signifi cantly more diff use damages than lungs of the 1:4 diluted-vaccine-injected group which showed no to mild infl ammatory changes. The semiquantitative scores of the diluted-vaccine-injected group and the undiluted-vaccine-injected group were 0.7 ± 0.3 and 1.9 ± 0.3, respectively by method one ([a street-view], mean ± SE, p = 0.027