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case_images: Schematic overview of the study design. Intravenous (IV) immunization with the BNT162b2 mRNA COVID-19 vaccine was performed following a two-dose regimen at 12 and 14 weeks of age, with spontaneous death of the animal two days following booster vaccination.
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case_images: The Netherlands) for use in an experimental study aimed to establish a mouse model of mRNA COVID-19 vaccine-induced myocarditis, as described earlier (approval nos. 2021-67 and 2022-68 (University of Antwerp Ethical Committee)) (see  Figure 1 ).
case_images: Necropsy examination of organs following spontaneous death. (A) A disproportional enlargement of several of the animal's major organs was observed at necropsy, including the liver, kidneys, spleen, and intestines (black arrows). (B) Animal with normal phenotype for reference.
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case_images: At necropsy examination, a disproportional enlargement of several major thoracic and abdominal organs was observed, including the liver, kidneys, spleen, lungs, and intestines (see  Figure 2 ).
case_images: Histopathological and immunohistochemical analysis of organs following spontaneous death. (A) At necropsy two days following BNT162b2 booster, heart, liver, kidneys, spleen, lungs, and skeletal muscle were collected and processed for hematoxylin and eosin (HE) staining. Histopathological examination of the organ sections revealed a diffuse and extensive infiltration by medium-sized atypical cells, often completely obliterating the normal parenchyma. The skeletal muscle section showed evidence of systemic leukocytosis without parenchymal abnormalities. (B) Immunohistochemical findings showing staining of medium-sized atypical cells positive for B-cell marker CD19 (left panel), with further nuclear staining positive for terminal deoxynucleotidyl transferase (TdT) (middle panel) and the c-MYC oncoprotein (right panel). Stainings are compatible with a B-cell lymphoblastic lymphoma (B-LBL) immunophenotype.
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case_images: Following organ dissection, tissues were formalin-fixed and paraffin-embedded, cut into 5 microm sections, and hematoxylin and eosin (HE) stained for histopathological examination (3801540BBE and 3801590BBE by Leica Biosystems, Wetzlar, Germany, see  Figure 3A ). For confirmation and further differentiation of the lymphoma diagnosis, immunohistochemistry (IHC) was performed using commercially available antibodies against CD19, terminal deoxynucleotidyl transferase (TdT) and c-MYC (see  Figure 3B ).
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case_images: Body weight evolution throughout the study and following BNT162b2 prime and booster. (A) During the full study course, the animal of this case report (red dotted line) was weighed weekly until primary vaccination and daily afterwards. Over the first five study weeks, the animal's body weight gradually increased reaching peak body weight. Afterwards, a drop in body weight was observed, without recovery. In contrast, all other animals belonging to the same experimental group (black lines) showed a progressive increase of body weight over time. (B) Similar to the other animals belonging to the same experimental group (black lines), the animal of this case report (red dotted line) showed a slight decrease in body weight following primary immunization. In contrast with the other animals, a remarkable 5.7% increase in body weight was observed following booster immunization.
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case_images: Over the next study weeks, the animal's body weight gradually increased reaching peak body weight at week five (i.e., one week prior to the first immunization) (see  Figure 4A ). The response to vaccination was similar to the other animals for the primary vaccination (slight decrease in body weight), in contrast to the booster vaccination (5.7% body weight increase in the animal versus slight decrease in most of the other animals) (see  Figure 4B ).