— Chinas leading vaccine manufacturer, Sinovac Biotech, has published its first tests of a vaccine against H5N1 bird flu, and the news is good.
The vaccine works at doses that are one-third to two-thirds lower than those used in the most successful trials published to date. The smaller the dose of vaccine virus needed to immunise someone, the better, as more doses can then be produced in a short time after the start of a pandemic.
The news is not entirely surprising. Sinovacs trials are the first published tests of a vaccine made of whole, dead H5N1 flu virus. New Scientist reported in December 2005 that scientists expect the whole virus to be more immunogenic than split viruses (see Tests dash hopes of rapid production of bird flu vaccine). But most vaccine companies have tested split-virus H5N1 vaccines because that is how they make ordinary flu vaccine.
In December 2005, French firm Sanofi reported on its vaccine, a split virus plus alum an immune-stimulating chemical, called an adjuvant. This vaccine did not elicit enough immunity to meet regulatory standards at doses containing less than 30 micrograms of the viruss main surface protein, haemagglutinin. Australian firm CSL reported in February 2006 that its vaccine, also a split virus with alum, worked at 15 micrograms, but in only half the people tested.
Just two 10-microgram doses of the Sinovac vaccine, also combined with alum, elicited regulatory-standard immunity in 65% of people six weeks after the trial began. And at least half of that regulatory-standard level of immunity which may not prevent illness but which might nevertheless offer life-saving protection developed in 96%.
In July, Belgium-based GlaxoSmithKline announced that its whole-virus vaccine worked at just 3.8 micrograms (see Bird flu vaccine breakthrough offers hope). But the details of this trial have not yet been published. The vaccine also used a novel, and secret, adjuvant not yet approved for use.
Whole-virus vaccines have another advantage: making a split virus destroys 20% to 30% of the vaccine virus which has to be laboriously grown in chicken eggs. Whole-virus vaccines avoid this waste.
The Chinese results mean the worlds existing vaccine manufacturing plants, working flat out for six months, could make enough vaccine for 742 million people. That rises to two billion with a 3.8 microgram dose. The problem with that is that six months after its start, a pandemic will probably be largely over.
Some vaccine experts are calling for an H5N1 vaccine that cross-reacts broadly with other, slightly different strains of H5N1 (see today's bird flu vaccines will have to do. That would mean a vaccine made of strains circulating before a pandemic can be stockpiled, and will still work to some extent against a pandemic strain. But there is no report yet on how broadly the vaccines tested so far cross-react.
Bird Flu - Learn more about the flu pandemic that could kill millions in our continually updated special report.
Journal reference: The Lancet (DOI: 10.1016/50140-6736(06)69294-5)
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