Fifteen-year-old Braden Wilson was frightened of Covid-19. He was careful to wear masks and only left his house, in Simi Valley, Calif., for things like orthodontist checkups and visits with his grandparents nearby.

But somehow, the virus found Braden. It wreaked ruthless damage in the form of an inflammatory syndrome that, for unknown reasons, strikes some young people, usually several weeks after infection by the coronavirus.

Doctors at Children’s Hospital Los Angeles put the teenager on a ventilator and a heart-lung bypass machine. But they could not stop his major organs from failing. On Jan. 5, “they officially said he was brain dead,” his mother, Amanda Wilson, recounted, sobbing. “My boy was gone.”

NEW DELHI — India, the unmatched vaccine manufacturing power, is giving away millions of doses to neighbors friendly and estranged. It is trying to counter China, which has made doling out shots a central plank of its foreign relations. And the United Arab Emirates, drawing on its oil riches, is buying jabs on behalf of its allies.

The coronavirus vaccine — one of the world’s most in-demand commodities — has become a new currency for international diplomacy.