In the constant search for a vaccine against tuberculosis — which now kills more people around the world than any other infectious disease — researchers have made an unusual discovery.

In tests on monkeys, they found that an almost century-old vaccine routinely given to infants in many countries is far more protective when injected into a vein rather than by the normal route, just under the skin.

Injecting the vaccine into a vein completely protected nine of 10 monkeys who were exposed to large doses of live TB germs six months later, according to the study. The research was led by scientists from the University of Pittsburgh’s medical school and the National Institute of Allergy and Infectious Diseases and published Wednesday by the journal Nature.

Although routine childhood vaccinations are not generally injected into a vein, an experimental malaria vaccine delivered that way has been successfully given to hundreds of children in Africa, so it is theoretically possible, the authors said.