Trained immunity refers to the fact that the innate immune system also demonstrates memory, resulting in a faster and more profound second innate reaction, days to weeks after a first reaction to another pathogen or vaccine. Thus, trained immunity is heterologous, non-specific. We applied this principle with MMR vaccination during the COVID-19 pandemic.In a prospective, observational, single-center study 255 subjects, most at high risk for infection with COVID-19, received preventive MMR vaccination; 36 got infected with COVID-19; all had a mild course, even though 40% had risk factors. This might in part be due to trained immunity, conveying innate immune memory secondary to MMR vaccination, enhancing the innate immune response once the subject gets infected with SARS-CoV-2.As a result the well-known immune suppression brought about by coronavirus might not work so well, as the innate immune system is primed, allowing the body to finally eliminate the virus more efficiently.