Measles, mumps and rubella vaccine and heterologous immunity: a way out of the COVID-19 crisis?

Heterologous immunity is a well-known concept in immunology wherein prior exposure to an antigen confers cross-protection against an unrelated antigen. With the surge in global COVID-19 cases, there has been significant research into the application of vaccine-induced heterologous immunity associated with measles, mumps and rubella (MMR) vaccine, Bacillus Calmette-Guérin vaccine, oral polio vaccine, and hepatitis A vaccine in curbing the worst outcomes of COVID-19 infection. Despite having specific vaccines against COVID-19, it is worthwhile exploring the application of available vaccines in the prevention of severe disease until the vaccines reach all sections of the population across the globe. In this article, we aim to outline the concept of heterologous immunity and its relevance in context to MMR vaccine and COVID-19.