Going now into its first human trials, a vaccine that could help to control a flu pandemic has shown encouraging results.

The vaccine, made by Acambis, based in Cambridge, should protect against all strains of influenza A, the type responsible for pandemics. Unlike existing vaccines it does not have to be reformulated each year to match the prevalent strains of flu, so it could be stockpiled and used as soon as a pandemic strain emerges. Nor does it need to be grown on fertilized chicken eggs, as the existing vaccines do, but can be produced by cell culture.

The results, announced yesterday by Acambis, show that in human volunteers the Acam-Flu-A vaccine was safe and produced an immune response against its target, a small protein (peptide) called M2e that is found on the surface of all A-strains of the flu virus. The vaccine was also tested on ferrets, which are commonly used in flu research because they are susceptible to human and bird flu.

The ferrets were divided into two groups and either vaccinated with the new vaccine or left unvaccinated. They were then exposed to a large dose of the H5N1 bird flu that has killed millions of chickens and more than 200 people across Asia since 2003. All the unvaccinated ferrets died, but 70 per cent of the vaccinated ones survived.

A significant problem with conventional vaccines is that they attack parts of the flu virus that can change rapidly. Each season the World Health Organization identifies the three strains that are circulating, normally two A-strains and one B, and the vaccine is made to order to provide protection against them. It is always a race against time, because millions of eggs have to be produced to grow the vaccine and if it is not used it is out of date by the following season.

Acambis’s approach was to identify some aspect of the virus that is unchanging. Pandemics are invariably caused by A-strains of flu; B-strains, which are found only in humans, may cause epidemics but have never caused pandemics.

The company identified a peptide, M2e, on the surface of all A-strains and developed a vaccine that targeted it. When an individual is vaccinated the vaccine teaches the immune system to recognize and be alert to the peptide so that as soon as flu arrives the body’s protective systems swing into action against it. To improve the vaccine’s effectiveness, it was combined in the trial with adjutants, chemicals that ginger up the immune system and improve its ability to learn. The adjuvant called QS-21, made by Antigenics, proved to be the best. When this was added, 90 per cent of those vaccinated had antibodies against the M2e peptide.

Michael Watson, the executive vice-president for research and development at Acambis, said: “If there was an immediate threat of pandemic flu, it would be possible to complete the trials and market the vaccine within three years. Without such a threat, it will likely take longer, perhaps five years.