LAUSR

SARS-CoV-2 has resulted in over 450 million confirmed cases since 2019. Although several vaccines have been certified by World Health Organization and are being vaccinated on a global scale, it has been reported that multiple SARS-CoV-2 variants can escape neutralisation by antibodies, resulting in vaccine breakthrough infections. Bacillus Calmette-Guérin (BCG) is known to induce heterologous protection based on trained immune responses. Here, we investigated whether BCG-induced trained immunity protected against SARS-CoV-2 challenge in the K18-hACE2 mouse model. Our data demonstrates that intravenous BCG vaccination induces robust trained innate immune responses and provides protection against wild-type SARS-CoV-2 as well as the B.1.617.1 and B.1.617.2 variants. Further studies suggest that myeloid cell differentiation and activation of the glycolysis pathway are associated with BCG-induced training immunity in the K18-hACE2 mice. Overall, our study provides the experimental evidence that establishes a causal relationship between intravenous BCG vaccination and protection against SARS-CoV-2 challenge.