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Mr Evans' father was among thousands of people to be infected by contaminated blood in the 1970s and '80s after being given a blood clotting agent called factor VIII.

Only 15% of people with haemophilia have haemophilia B. Most have haemophilia A—a genetic disorder caused by a deficiency in a different blood-clotting protein called factor VIII, which is encoded by a different gene.

By the mid 1970s, a new treatment for haemophilia, known as factor VIII/IX, became available for the first time.

Each patient was given a single injection of recombinant factor VIII, followed by a washout period of at least 3 days before an injection of the fusion protein BIVV001.

Those with the condition are either missing or low on a clotting protein called factor VIII, or FVIII.