COVID-19 infections have presented with a very unusual morbidity penetration, where patients younger than 50 show little morbidity from the disease, with mortality dramatically increasing above age 50. This is a very different presentation from other viral diseases, suggesting that some factor is protective in younger people, and missing in older patients. Theory was hypothesised that different exposure to vaccines between younger and older people may account for this different morbidity rate. Widely deployed measles-rubella containing vaccines (MRCV) including MMR, MR, and MMRV are believed to be why children, teenagers and other young adults often have few symptoms from COVID-19, and few deaths are attributed to COVID-19 in the young.
Infants are presumed well protected from COVID-19 because their own mothers have mostly likely had two MRCV vaccinations, thus passing along MRCV related passive immunity to them. Ordinarily, babies receive their first dose of the measles, mumps and rubella (MMR) vaccine in our country between 9-12 months of age. A second dose of MMR is recommended between ages 15-18 months.
The efficacy of MRCV has been shown to go down with age, leaving some of those who received the vaccines in their youth more vulnerable as they age. Most people over age 60 never received any form of MRCV.
Live measles vaccine has previously been considered in studies as a base for other Coronavirus vaccines including SARS; novel alphacoronaviruses and paramyxoviruses (the measles family) have been found to cocirculate; and MRCV have previously been shown to
generally increase immunity against many unrelated viruses. An excellent discussion was presented noting the homology of amino acid sequences between the COVID virus, and the rubella virus, possibly explaining cross-over
reactivity of the vaccines.