Most drugs can be used for more than just the disease they were initially designed to treat.

For example, disulfiram (brand name Antabuse) was developed to treat parasites but has since been found to be useful for treating alcoholism. It’s much quicker and cheaper to ‘repurpose’ drugs in this way than it is to develop them from scratch, which is why we now have dexamethasone — a steroid developed in the late 1950s — for treating severe cases of COVID-19.

But what about vaccines? Surely they are designed to only protect against a specific disease (or diseases, for combined vaccines)?

So why are researchers studying BCG (the vaccine against tuberculosis) and MMR (measles, mumps and rubella) as a possible protection against COVID-19?