Scientists in the UK have begun testing the BCG vaccine, developed in 1921, to see if it can save lives from Covid.
The vaccine was designed to stop tuberculosis, but there is some evidence it can protect against other infections as well.
Around 1,000 people will take part in the trial at the University of Exeter.
But while millions of people in the UK will have had the BCG jab as a child, it is thought they would need to be vaccinated again to benefit.
Vaccines are designed to train the immune system in a highly targeted way that leaves lasting protection against one particular infection.
But this process also causes wide-spread changes in the immune system. This seems to heighten the response to other infections and scientists hope it may even give our bodies an advantage against coronavirus.
- Previous clinical trials have shown the BCG jab reduced deaths by 38% in newborns in Guinea-Bissau, mostly by reducing cases of pneumonia and sepsis.
- Studies in South Africa linked the vaccine to a 73% reduction in infections in the nose, throat and lungs; experiments in the Netherlands showed BCG reduced the amount of yellow fever virus in the body.
"This could be of major importance globally," Prof John Campbell, of the University of Exeter Medical School, told the BBC.
"Whilst we don't think it [the protection] will be specific to Covid, it has the potential to buy several years of time for the Covid vaccines to come through and perhaps other treatments to be developed."
The UK trial is part of the international Brace-study, which is also taking place in Australia, the Netherlands, Spain and Brazil, recruiting 10,000 people in total.
It will focus on health and care workers, as they are more likely to be exposed to coronavirus, so researchers will know more quickly if the vaccine is effective.
Sam Hilton, a GP from Exeter, is taking part in the trials since, as a doctor, he is at higher risk of catching Covid.
"There's quite a good theory BCG might make you less likely to get unwell when you get Covid," he told the BBC.
"So I see it as a potential for me to get protected a bit, which means I'm more likely to come to work this winter."
Dr Tedros Adhanom Ghebreyesus, the director general of the World Health Organization, is one of the authors of a Lancet article saying the BCG vaccine has the potential to "bridge the gap before a disease-specific vaccine is developed".
"This would be an important tool in the response to Covid-19 and future pandemics," the article states.
However, the BCG vaccine will not be a long-term solution.
Any enhanced resilience to Covid is expected to wane meaning people who were immunised with BCG in childhood would no longer have protection. BCG has not been routinely used in the UK since 2005 because levels of tuberculosis are so low.
Additionally, the vaccine will not train the immune system to produce the antibodies and specialist white blood cells that recognise and fight off the coronavirus.
The big goal remains a vaccine that specifically targets the coronavirus. Ten such vaccines are in the final stages of clinical research, including the one developed at the University of Oxford.
Prof Andrew Pollard, from the Oxford Vaccine Group, told the BBC: "The way that most vaccines work is to make a very specific immune response against the germ you are trying to prevent.
"But in order to make a good immune response, there is also a rather non-specific 'souping-up' of the immune response and that changes the way the immune system is able to respond in the future.
"The problem we have today is I can't tell you what you could do with other vaccines to try to improve your ability to respond to coronavirus because we have no evidence at all."
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