As soon as he tested positive, contact-tracers brought his whole family, including his 33-year-old mother, his 40-year-old father and his three younger brothers, aged 8, 5 and two months, in for observation and testing.

The family also became the first participants in a clinical trial of a new Ebola vaccine, said Dr. Emily Kainne Dokubo, an epidemiologist at the U.S. Centers for Disease Control and Prevention who was then the leader of the agency’s Ebola response in Liberia and is the lead author of the Lancet study.

All 120 people with any recent contact with the family were vaccinated, she said. None fell ill, and that helped prove the Merck vaccine works.

This year, the vaccine was used to defeat the most recent Ebola outbreak, which took place in the Democratic Republic of Congo. About 3,200 people were vaccinated, and new cases faded out after only about three months. On Tuesday, the director-general of the World Health Organization will officially declare it over.

In Liberia, after the 15-year-old died, blood tests showed that his father and 8-year-old brother had Ebola. With treatment, both recovered.

The 5-year-old apparently was never infected.

Neither the mother nor the new baby had virus in their blood but, rather mysteriously, both had antibodies against it. That suggested the mother had had an earlier infection and that the baby had absorbed protective antibodies through breast-feeding.