Why is a person with diphtheria injected with an anti-diphtheria serum and not a vaccine?

Serum and vaccine are drugs that work on the basis of different components. Vaccination is associated with the introduction of fragments of already dead viruses to a person, which make it possible to form persistent immunity to a specific infection. The serum contains living fragments that act almost instantly and kill the disease that a person has contracted. In the case of diphtheria, the patient needs immediate treatment as it progresses very quickly. Therefore, the choice is made in favor of whey.