The measles-mumps-rubella (MMR) vaccine has been theorized to provide protection against COVID-19. Our aim was to determine whether any MMR IgG titers are inversely correlated with severity in recovered COVID-19 patients previously vaccinated with MMR II. We divided 80 subjects into two groups, comparing MMR titers to recent COVID-19 severity. The MMR II group consisted of 50 subjects who would primarily have MMR antibodies from the MMR II vaccine, and a comparison group of 30 subjects who would primarily have MMR antibodies from sources other than MMR II, including prior measles, mumps, and/or rubella illnesses. There was a significant inverse correlation (rs = -0.71, P < .001) between mumps titers and COVID-19 severity within the MMR II group. There were no correlations between mumps titers and severity in the comparison group, between mumps titers and age in the MMRII group, or between severity and measles or rubella titers in either group. Within the MMR II group: mumps titers of 134 to 300 AU/ml (n=8) were only found in those who were functionally immune or asymptomatic; all with mild symptoms had mumps titers below 134 AU/ml (n=17); all with moderate symptoms had mumps titers below 75 AU/ml (n=11); all who had been hospitalized and required oxygen had mumps titers below 32 AU/ml (n=5). Our results demonstrate that there is a significant inverse correlation between mumps titers from MMR II and COVID-19 severity.