The BCG vaccine offers more protection against tuberculosis than experts currently appreciate, say scientists.
TB kills at least 2m people a year
A UK study suggests that BCG protects some people in a way previously unknown - by stopping them catching the infection in the first place.
Experts had thought it merely worked by preventing TB infection turning into an active disease.
The Oxford University researchers told the Lancet the findings could help with new vaccine development.
TB kills at least two million people a year globally, or 5,000 every day, mainly in the developing world, according to the World Health Organization.
Although BCG helps prevent TB, it is not 100% effective.
Some trials have suggested protection could be as low as 30%.
Scientists are currently working to find new vaccines with improved efficacy.
Until recently, doctors have relied on the century-old tuberculin skin prick test or heaf test to check for TB infection.
It can take three to five days to give a result and can give false-positive results in people who have been vaccinated against TB, or exposed to other similar types of microbes.
It is thought up to half of people given treatment for TB on the basis of a positive skin test do not have TB at all. Similarly, the test fails to detect half of people with latent TB.
In their study, Dr Ajit Lalvani and colleagues used a different test called the ELISpot to assess infection, which does not give a false-positive result if a person has received BCG, unlike the heaf test.
They tested 979 children from Istanbul, Turkey, who had shared a household with at least one adult with TB and therefore could have caught the infection.
Of these children, 770 had a BCG scar, indicating that they had been vaccinated against TB.
When the researchers looked at the ELISpot test results, they found absence of a BCG scar was a strong, independent risk factor for TB infection while the presence of a BCG scar was associated with a 24% reduction in risk of being infected.
Dr Lalvani said: "Our findings show that children can be protected against TB infection by vaccination.
"Until now, managing TB has always been a two-pronged approach, preventing disease progression from latent to active TB and treating patients with active TB.
"Now we know we can protect against infection - so it's a three-pronged approach."
He said that the work should help with new vaccine development.
Dr John Watson, head of the Respiratory Diseases Department at the Centre for Infections for the Health Protection Agency, said: "This study sheds some light on this complex issue."
Professor Philip Marsh, also of the Health Protection Agency, said: "There is currently a global effort to produce an improved vaccine that will provide even better protection against tuberculosis than that conferred by BCG.
"These new vaccines are either based on improving BCG itself or by using defined molecules (sub-units) to boost the protection already provided by earlier BCG vaccination.
"The Health Protection Agency is actively participating in the pre-clinical studies to evaluate these new vaccine approaches."
Scientists at the National Institute of Medical Research in the UK have been developing a candidate TB vaccine.
In animals, it appears to switch off a subversive mechanism TB uses to evade destruction by the immune system and thrive in parts of the body such as the lung.
Other researchers at Oxford University have tested a vaccine in humans that they believe could boost the power of the existing BCG vaccine.
In July, the UK government decided to stop immunising children aged 10-14 against TB because the infection risk is small in this population.
Based on the latest UK data, the Department of Health says identifying and vaccinating those who are most likely to catch the disease - babies and older people - is the most effective policy.