A vaccine against the H5N1 strain of bird flu, developed by Sanofi Pasteur, was effective in a clinical trial but only at the highest dose tested, sharply reducing the number of people who could be protected using existing stockpiles.

The 451-patient study included a range of doses of the vaccine candidate – from 7.5mcg to 90mcg – but only the highest dose were antibodies produced in quantities sufficient to protect against bird flu.

“The need for a vaccine with a total dose of 180mcg would pose a considerable barrier to rapid production of a supply that would be adequate to meet the world's requirements should a pandemic occur,” they said. One solution could be to include the use of an immune response-boosting adjuvant such as alum in the vaccine formulation.

In December, Sanofi Pasteur said another trial suggested two 30mcg doses of the vaccine given with an alum was able to produce an immune response comparable to that achieved with current vaccines used to protect against seasonal flu.

The researchers also noted they had set the threshold for an effective immune response quite high in the current study, and it was possible a lower antibody response could still be effective.

The USA has spent more than $150 million laying down supplies of the bulk antigen used in Sanofi Pasteur’s H5N1 vaccine, and has set itself a target of having enough to vaccinate 20 million people. But the current data suggest stocks will only be enough to dose 4 million people, according to Anthony Fauci, the director of the US National Institute of Allergy and Infectious Diseases.

Avian flu has infected 186 people in eight countries and killed 105, according to the latest World Health Organisation figures.

A number of other companies are working on the development of pandemic flu vaccines, in most cases based on the H5N1 strain, including GlaxoSmithKline, Chiron, Akzo Nobel, MedImmune and Baxter International.

GSK vaccine starts trials

GSK said today it had started clinical trials of two H5N1 vaccines in humans and was on track to have a candidate in production by the end of the year.

A German trial is testing a vaccine that includes alum, while a second study in Belgium is looking at a vaccines incorporating a novel adjuvant. Preliminary results from the trials are expected in the third quarter.

- Meanwhile, GSK reported that its inhaleable flu drug Relenza (zanamivir) has been approved in the USA for the prevention of influenza in adults and children aged five years and older. The drug was already registered for treating flu, and the approval puts it on level terms with Roche’s oral flu drug Tamiflu (oseltamivir). Fears of a pandemic drove sales of Tamiflu to 1.6 billion Swiss francs ($1.2bn) last year, around half of which came from government orders.