Millions of asthmatics and hay fever sufferers could be spared the misery of severe attacks by a new vaccine, which has been successfully tested on people with an allergy to cats.

The vaccine has already shown promising results when studies at the National Heart and Lung Institute at Imperial College, London, found that the allergic reactions in patients were reduced by half.

These initial results suggests that up to three million people who suffer from allergy-based asthma, and millions of hay fever sufferers, will have a long-lasting vaccine to protect them against attacks for the first time.

The new drug, which should be available within five years, will give people protection for several years just by taking a short course of treatments. Efforts are now being turned to making the drug work for people with dust mite, pollen or grass allergies.

Experts believe it will be particularly useful for asthmatics who do not respond well to conventional steroid-based treatments. But many current asthma steroid drugs also have to be taken all the time, while other treatments for asthma and hay fever are used only once an attack has started.

The plight of Britain's 5.2 million asthmatics has been the focus of a year-long campaign by The Independent on Sunday, which has highlighted the growing demand for safe, workable treatments.

The institute's findings are being presented at a major conference on new treatments for asthma, organised by the Royal Society of Medicine, where other experts will call on ministers to greatly improve the help given to asthmatics by the Health Service.

The vaccine is based on the well-tested but occasionally risky technique of immuno-therapy, where the body is given small doses of an allergy-causing substance in order to stimulate the body's natural defences. The new drug uses modern laboratory techniques to "cut up" the genes and counter its allergic effects.