As scientists race to make a new vaccine against covid-19 , some are touting a century-old one as a possible stopgap.

The vaccine is BCG (Baccillus Calmette-Guérin), which protects against tuberculosis (TB). In countries with routine BCG vaccination, the pandemic appears to be less severe. Advocates of BCG say this could be because the vaccine is a general immune-system enhancer – a long-standing but unproven hypothesis. But critics say we can’t be sure.

BCG has been used since 1921. It contains a live bacterium related to the one that causes TB, …