For the billions of potential victims of Aids and malaria in the developing world, a reliable vaccine would be the holy grail. But for the pharmaceutical or biotech company that discovers it, it could be a poisoned chalice.

Owen Barder, who leads a public health programme at the US Center for Global Development, a think-tank, says: “One representative from the pharmaceutical industry told us that in some ways, actually discovering a vaccine for Aids was their biggest nightmare.”