The first vaccine against avian flu for the U.S. was endorsed by a Food and Drug Administration advisory panel, but merely as a stopgap measure until better vaccines are developed.
The experimental vaccine, made by Sanofi-Pasteur, protected only 45% of the 91 people on whom it was tested in a hasty clinical trial. Reaching even that level of protection required 12 times the dose of antigen delivered by a typical flu shot, and it had to be given in two shots several weeks apart. It is also based on a virus strain that was circulating in Vietnam three years ago.
Robert Couch, an advisory panel member from Baylor College of Medicine in Houston, said members knew of better vaccines still in development, but this is the only vaccine we had in front of us.
The panel voted unanimously that the vaccine was effective enough to be used. The F.D.A. usually accepts the recommendations from its expert panels.
A spokesman for Sanofi, a unit of Sanofi-Aventis, described the move as a positive first step.
Because it would be unethical to infect trial subjects deliberately with a flu that can kill, it is not possible to say accurately how many people the vaccine can save from death or hospitalization, so 45% protection is a bit of a misnomer, the spokesman, Len Lavenda, said.
The closest scientists can get is to draw blood from subjects a month after vaccination and see how many have developed as many antibodies to the target virus as are provoked by a seasonal flu shot to its targets.
The government already has plans to stockpile enough avian flu vaccine to protect 20 million doctors, nurses, paramedics, laboratory technicians and other emergency workers against the H5N1 strain if a pandemic breaks out. Sanofi does not plan to sell this vaccine to the public; it is developing newer ones, including one based on a later strain from Indonesia, Lavenda said.
The World Health Organization said recently that 16 companies from 10 countries were developing prototype vaccines, and more than 40 clinical trials had been finished or were under way.
So far, all have been found to be safe. The difficulty, experts said, is finding one that works at low doses and can be made quickly. Most now must be grown in eggs; the weakened virus must be injected and the eggs cracked by hand to remove the liquid for purifying. At present, it would take months to create enough for the whole country.
Some vaccines being tested now can be grown in a broth of cells; others have additives that boost the immune system, helping lower doses work.
Robert Webster, a virologist at St. Jude Childrens Research Hospital, has suggested that the current seasonal flu vaccine, which has an H1N1 strain in it, may provide some protection against an H5N1 pandemic because there could be overlap in immune response to the N1 component of both viruses.