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Research published today (11 May) indicates that a highly efficient vaccine capable of protecting large numbers of people against the H5N1 virus remains elusive.

The results of the latest vaccine trial are more promising than those published in March by researchers at the US-based University of Rochester (see 'Disappointing' results from US bird flu vaccine trial).

But they deal a blow to the theory that adding chemicals called adjuvants to H5N1 vaccines could significantly increase their effectiveness and help solve the limited global capacity to produce the vaccines (see Pandemic flu: fighting an enemy that is yet to exist).

The latest trial, led by Melanie Saville of Sanofi Pasteur and published by The Lancet, compared the effectiveness of an aluminium hydroxide adjuvant in vaccines of three doses: 7.5, 15 and 30 micrograms.

The most effective treatment was a 30-microgram dose containing the adjuvant and administered twice, 21 days apart.

This treatment triggered a protective immune response in 67 per cent of trial participants, meaning it met European Union standards of protection for regular seasonal flu vaccines.

It is still unclear how effective the vaccines currently being developed would be if H5N1 changed and sparked a human flu pandemic.

Adding an adjuvant to the 30-microgram dose boosted the proportion of protected people from 52 to 67 per cent. But the adjuvant had no effect on the 15-microgram vaccine and appeared to lessen the effectiveness of the 7.5-microgram dose.

In an accompanying commentary, two US flu vaccine researchers say the results suggest that adjuvants are "only a partial solution — one that would still limit the number of vaccine doses available given the existing global manufacturing capacity".

Reference: The Lancet doi:10.1016/SO140-6736(06)68656-X

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