Table 1.

Summary of the data available on the induction of heterologous immunity against COVID-19 by MRCVs.

Authors	Study population	Type of study	Result
Sidiq et al. [23]	Children	Hypothesis	30 amino acid sequence homologies between measles, rubella and SARS-CoV-2 proteins could lead to cross-protection to COVID-19 after MMR immunisation
Anbarasu et al. [24]	Children	Hypothesis	Induction of NK cells and IFNs after MMR immunisation could boost natural immunity against COVID-19
Saad and Elsalamony [25]	All population	Hypothesis	Measles immunisation could lead to milder COVID-19 infections by generation of heterologous immunity
Ashford et al. [28]	All population	Hypothesis	MMR vaccination may provide strong protection to COVID-19 particularly in the elderly and those with comorbidities
Larenas-Linnemann and Rodríguez-Monroy [30]	All population	Clinical trial	Out of 255 subjects vaccinated with MMR vaccine, only 36 participants developed COVID-19 and had a mild course of disease
López-Martin I et al. [31]	Primary care physicians	Observational	67.92% of the physicians infected with COVID-19 who had received MMR vaccine presented with mild symptoms and did not require hospitalisation
Gold et al. [33]	All population	Observational	There was a significant inverse correlation between mumps titres in MMR vaccinated subjects and COVID-19 severity
Lundberg et al. [34]	Healthcare workers	Case control	Protective effect of MMR vaccination against COVID-19 found in males
Hassani et al. [35]	COVID 19-infected patients	Case control	Measles vaccination triggers production of cross-reactive antibodies against COVID-19
Pawlowski et al. [36]	All population	Observational	MMR vaccination has a strong negative correlation with COVID-19 infection rates
Gujar et al. [37]	Children	Case control	MCVs have a strong protective effect in paediatric age group against COVID-19 infection
Mysore et al. [38]	All population	Observational	High correlation between T-cell responses to COVID-19 and MMR vaccination in COVID-19 recovered and immunised individuals
Ogimi et al. [39]	All population	Ecological	Association between MCV coverage and fewer reported COVID-19-related deaths which did not hold when adjusted for HAQI
Kandeil et al. [40]	Mice	Animal trial	No cross-reactive antibodies against COVID-19 seen in MMR vaccinated mice
Fidel et al. [46]	All population	Hypothesis	MMR vaccination through induction of trained immunity could provide a low-risk, high-reward protection against COVID-19

HAQI, Healthcare Access and Quality Index; MCVs, measles-containing vaccines; MMR, measles, mumps and rubella.