In our study we used Quest Diagnostics (Test Code 5259) for checking MMR titers including mumps. These same immunoassays may also be available from other labs. Specifications for these are detailed in the methods section of our titer paper. Different immunoassays may not result in titer levels correlating the same way they did in our study since each immunoassay measures titers differently.
MMR II (known as MMRVaxPro in Europe) is the only widely available MMR vaccine in the United States. It is manufactured by Merck and was initially licensed in 1979. It includes the Edmonston strain of measles, the Jeryl Lynn (B-level) strain of mumps, and the Wistar RA 27/3 strain of rubella. There are numerous variations in virus strains contained in MMR vaccines available in other countries. It is not known whether any of these other MMR vaccines produce mumps antibodies which may also be associated with COVID-19 severity.
Mumps antibodies from MMR II are exclusively from the Jeryl Lynn strain of mumps. Those born outside the United States, those born more than 42 years ago, or anyone who contracted mumps naturally as a disease may have antibodies from strains other than the Jeryl Lynn strain. Antibodies from natural disease did not correlate with COVID-19 severity in our study. This does not mean these other mumps antibodies are definitively not protective. It just means the mumps titer tests we used could not predict COVID-19 severity in those born in the U.S. over 42 years of age, those born in other countries where mumps may have been common as a natural disease, or in individuals who may have had other versions of the MMR vaccine.
While mumps titers were the only titers we found that significantly and inversely correlated with COVID-19 severity, this does not mean mumps antibodies are the only possible source of protection from MMR II vaccination. It is also possible that mumps titers are simply an accurate proxy measure of the general strength remaining of all antibodies from MMR II vaccinations, and that measles and/or rubella antibodies may play an important part in addition to or instead of mumps antibodies. Even if measles or rubella antibodies are protective against COVID-19, this may not be able to be determined through simple titer tests. This is explained further in our current paper.