Tuberculosis can be transmitted even through something as simple as a cough on a crowded bus. But the people at the highest risk include family members of patients with active tuberculosis, the doctors and nurses caring for them and, in countries where tuberculosis is common, people living or working in crowded conditions, such as prisoners and miners.

But in any country, people are also at risk of infection if they have H.I.V., are severely malnourished, are taking immune-suppressive cancer chemotherapy or organ-transplant drugs, have diabetes or are on dialysis.

The new study, however, tested the vaccine only in people who were H.I.V.-negative and whose blood tests showed they had latent tuberculosis.

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But at least a quarter of the world’s population would come up positive for latent tuberculosis on a blood or skin test. The result means only that they have been exposed to tuberculosis germs some time in the past.

“We have no idea if they have been infected last month or 20 years ago,” Dr. Raviglione said. Those infected long ago may have already have cleared their bodies of the infection.

Most people who are ever going to develop active tuberculosis do so within two years of their first infection. Therefore, some prominent researchers argue that latency tests greatly exaggerate the number of people at risk.

As a result, relying on them would cause many more people to be vaccinated than could benefit.

Dr. Lalita Ramakrishnan, a tuberculosis expert at the University of Cambridge in Britain, noted that participants in the vaccine study were less likely to develop active tuberculosis in the first year than in the second.