A man and a woman walk into a doctor’s office. She has a cat allergy; he has a cat. “They say, ‘You’ve got to do something or we can’t get married,’” says Michael Blaiss. It sounds like the beginning of a bad joke, but it’s actually a typical day at Blaiss’s private allergy practice in Memphis, Tennessee. People often face tough decisions when a loved one cannot cohabit with feline companions.

Cat dander—microscopic pieces of dry cat skin coated with Fel d 1, a protein responsible for most cat allergies that is secreted by cat glands onto the skin and transferred to fur from cat saliva through grooming—elicits a reaction in an estimated 17% of individuals in the US. Antihistamines and steroids can dull the symptoms, but the only disease-modifying therapy currently available is a series of injections made of cat dander extract, a soup of proteins literally washed from cat fur and bottled. Whole allergen treatment is time consuming, involving some 30–80 shots over three to five years, and risky, with the chance of rare life-threatening anaphylactic reactions to the injections.

An Oxford, UK–based company, Circassia, hopes to change all that with its new ToleroMune cat allergy vaccine, a molecular approach to the problem. The vaccine is made of seven synthetic peptides, each only 15–20 amino acids long and derived from Fel d 1. The carefully selected peptides quiet the immune system’s aberrant T cell response but avoid activating mast cells, which cause allergic reactions and anaphylaxis.

“We know exactly what is in every vial,” says Steve Harris, Circassia’s chief executive.

In a recently published phase 2 study, 21 people received four injections of the therapeutic vaccine over a three-month period in which they were exposed to cat dander. A year after starting the treatment, these individuals showed a significantly greater reduction in nose- and eye-related symptoms than 29 participants who received a placebo and the same allergen hazard.

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