In the recent paper by Gold et al. (2020, mbio.asm.org), an inverse relationship is seen between mumps immunity (IgG titers) and COVID-19 severity. Those individuals who were considered to be "functionally immune" to COVID-19 showed the highest IgG titers and as symptoms progressed through asymptomatic, mild, moderate and severe patients, these titers dropped consistently.
The "severe" group, consisting of four individuals, had an average mumps titer of 8 AU/mL, which functionally falls below the level required even for mumps immunity. It should be noted that all 50 participants in this portion of the study had received at least one MMR II (Merck) vaccine.
From this, the authors of the paper suggest that the MMR II vaccine is protective against severe cases of COVID-19 and specifically state, "The signi?cance of our study is that it showed that mumps titers related to the MMR II vaccine are signi?cantly and inversely correlated with the severity of COVID-19-related symptoms, supporting the theorized association between the MMR vaccine and COVID-19 severity." No correlation was seen between COVID-19 severity and measles or rubella titers.