Unprecedented immunization campaigns have been rolled out worldwide in an attempt to contain the ongoing COVID-19 pandemic. Multiple vaccines were brought to the market, among two utilizing novel messenger ribonucleic acid technology. Despite their undisputed success in decreasing COVID-19-associated hospitalizations and mortality, various adverse events have been reported. The emergence of malignant lymphoma is one of such rare adverse events that has raised concern, although an understanding of the mechanisms potentially involved remains lacking. Herein, we present the first case of B-cell lymphoblastic lymphoma following intravenous high-dose mRNA COVID-19 vaccination (BNT162b2) in a BALB/c mouse. Two days following booster vaccination (i.e., 16 days after prime), at only 14 weeks of age, our animal suffered spontaneous death with marked organomegaly and diffuse malignant infiltration of multiple extranodal organs (heart, lung, liver, kidney, spleen) by lymphoid neoplasm. Immunohistochemical examination revealed organ sections positive for CD19, terminal deoxynucleotidyl transferase, and c-MYC, compatible with a B-cell lymphoblastic lymphoma immunophenotype. Our murine case adds to previous clinical reports on malignant lymphoma development following novel mRNA COVID-19 vaccination, although a demonstration of direct causality remains difficult. Extra vigilance is required, with conscientious reporting of similar cases and a further investigation of the mechanisms of action explaining the aforementioned association.

Addendum: B-cell lymphoblastic lymphoma following intravenous BNT162b2 mRNA booster in a BALB/c mouse: a case report

The mechanism underlying myopericarditis associated with mRNA COVID-19 vaccination, including increased susceptibility in young males, remains poorly understood. This study aims to explore the hypothesis that engaging in physical exercise at the time of mRNA COVID-19 vaccination may promote a cardiac inflammatory response, leading to the development of myopericarditis. Male BALB/c mice underwent treadmill running or remained sedentary for five weeks. Subsequently, two doses of the Pfizer/BioNTech vaccine or vehicle were administered with a 14-day interval, while the exercise regimen continued. The animals were euthanized days after the second vaccination. Vaccination was followed by body weight loss, increased hepatic inflammation, and an antigen-specific T cell response. Small foci of fibrovascular inflammation and focal cell loss were observed in the right ventricle, irrespective of vaccination and/or exercise. Vaccination did not elevate cardiac troponin levels. Cardiac tissue from the vaccinated mice showed upregulated mRNA expression of the genes IFNγ and IL-1β, but not IL-6 or TNFα. This pro-inflammatory signature in the heart was not exacerbated by endurance exercise. Ex vivo vascular reactivity remained unaffected by vaccination. Our data provide evidence for the cardiac safety of mRNA COVID-19 vaccination. The role of exercise in the development of pro-inflammatory cardiac changes post mRNA vaccination could not be established.

Athletes often exhibit unexplained non-ischemic myocardial fibrosis, which is associated with malignant arrhythmias. Given the prevalent (over)use of NSAIDs among athletes and their harmful effects during viral myocarditis, this study examined the effects of combined NSAIDs and endurance exercise on the disease progression. To this end, male C57BL/6J mice underwent eight weeks of treadmill running (60 min/day; 18 cm/s) or no exercise. After two weeks, mice were implanted with mini-pumps delivering ibuprofen (70 mg/kg bw/day) or vehicle. Myocarditis was induced via intraperitoneal coxsackievirus inoculation. Mice were sacrificed six weeks post-inoculation for ventricular arrhythmogenicity evaluation and cardiac histopathological and molecular analysis. Exercising coxsackievirus-infected mice receiving ibuprofen recovered faster from weight loss. Mortality was low and similar across groups. Histopathology revealed abating inflammation and cell loss, without significant group differences. While exercise tended to increase extensive myocardial fibrosis, statistical analyses indicated no significant differences—with or without NSAIDs—in perivascular fibrosis, interstitial fibrosis, or myocardial scarring. NSAIDs—irrespective of exercise—did not increase arrhythmogenicity. In conclusion, ibuprofen in exercising mice with viral myocarditis resulted in faster weight loss recovery, without significant differences in inflammation, fibrosis, or arrhythmogenesis compared to exercise-only mice.