To the editor

We would like to report here on our clinical observations in 212 subjects, vaccinated in our Center since the start of the Coronavirus disease-2019 (COVID-19) pandemic, with the mumps-measles-rubeola (MMR) vaccine and of whom thirty-two have presented COVID-19, all with a remarkably mild course.
In the light of the COVID-19 pandemic, observing the highly contagious and virulent nature of the virus, new to mankind and for which no actual treatment nor vaccination exists, we have been searching for methods to enhance innate immunity. Moreover, the pandemic started in our country just after a rise in measles cases had motivated the Ministry of Health to recommend measles re-vaccination. Aware of the existence of trained immunity we decided to apply this concept and from March 2020 onward recommend MMR vaccination, but with extra emphasis among family members of COVID-19 cases. In June 2020 the American Society for Microbiology (AMS) speculated in a press-release that “the MMR vaccine could serve as a preventive measure to dampen …. COVID-19 infection.”

In a prospective observational trial we followed MMR vaccinated subjects searching for COVID-19 cases. All patients were vaccinated subcutaneously with 0.5mL of the MMR vaccine containing live-attenuated virus (≥1,000 CCID50 of measles, ≥5000 CCID50 of mumps and ≥1000 CCID50 of Rubella virus) and follow-up was given by (bi)monthly phone calls or contact via electronic media. COVID-19 infection was considered confirmed with a positive result of the SARS-CoV-2 reverse transcription polymerase chain reaction (RT-PCR), the detection of SARS-CoV-2 specific antibodies or the combined presence of a direct contact with a confirmed case plus anosmia/ageusia plus at least two classic symptoms. Direct contact with a confirmed case, accompanied by classic symptoms, but without olfactory nor gustatory alterations were considered highly probable cases. We graded the clinical severity of COVID-19 on a simplified scale we considered more suitable in an out-patient setting, see table 1.

Among the 212 vaccinated subjects there are 22 confirmed and 10 (highly) probable COVID-19 cases, twelve of them with hypertension, diabetes, obesity, smoker or uncontrolled asthma as possible risk-factors. All had minor respiratory symptoms at most. As people are generally very reluctant to go to a laboratory or take a chest X-ray, we have installed close follow-up in probable positive cases with pulse oximetry and home peak-expiratory-flow (PEF) measurements; only one uncontrolled asthmatic had one day hypoxemia. All received general supportive measures and the policy toward fever was permissive, keeping paracetamol use to a minimum. Some received off-label high-dose ivermectin the first two days. None presented respiratory insufficiency to the degree of needing oxygen.
Table 1. Cases of COVID (confirmed or highly probable) within weeks of MMR vaccination, COVID severity compared with case fatality rates for Mexico per age-sex group and per co-morbidity.