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updated 5/15/2013 12:20:07 PM ET 2013-05-15T16:20:07

Although the rise in cases of the H7N9 bird flu in China seems to have slowed, health officials are developing a vaccine against the virus, and will test it in a clinical trial, experts say.

But how will they know the vaccine works?

The first step is to make a vaccine. Researchers at the Centers for Disease Control and Prevention (CDC) have already made a preliminary " seed vaccine" against strains of the H7N9 virus that have been found in infected people. Earlier this month, the CDC offered to begin shipping this vaccine to manufacturers that wanted to start creating batches to be used in testing.

Next, volunteers in the United States will be recruited for the trial, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. During the trial, health officials will assess the vaccine's safety (such as whether there are any adverse reactions).

They will also look at the strength of immune response the vaccine induces in people, to see whether the vaccine could successfully protect a person against infection, Fauci said. The NIH will oversee these trials.

There has been some concern that a vaccine against H7N9 might not induce an immune response strong enough to be effective. Studies of vaccines against viruses in the H7 family have suggested that even two vaccine doses do not produce an adequate response, Michael Osterholm, director of the University of Minnesota's Center for Infectious Diseases Research and Policy, wrote in an editorial in the Journal of the American Medical Association on May 9.

Some vaccines contain ingredients called adjuvants, which boost their effects. Although adjuvants have not been added to flu vaccines in the United States in the past, the clinical trials of the H7N9 vaccine will include one version with an adjuvant, and one without, Faucitold LiveScience.

Because no cases of H7N9 have been reported in the United States — and because it would be unethical to expose people to the virus just to test the vaccine — the trials cannot directly assess the effectiveness of the vaccine in preventing H7N9 infection.

However, researchers will indirectly test the vaccine's effectiveness by taking blood from vaccinated people, and checking to see if antibodies in their blood can neutralize (or kill) the H7N9 virus in lab dishes, Faucisaid.

Similar trials have been conducted in the past for the H5N1 vaccine, Fauci said.

Health officials are not yet sure whether they will need to use the H7N9 vaccine. The decision to produce large quantities of the vaccine and vaccinate the public will depend on whether H7N9 turns into a global disease outbreak, Faucisaid.

Right now, cases of H7N9 have not been seen outside China and Taiwan, and there have been no reports of sustained human-to-human transmission of the virus. As of May 8, health officials knew of 131 people who had been infected with H7N9, of whom 32 had died, according to the World Health Organization.

Follow Rachael Rettner@RachaelRettner. Follow MyHealthNewsDaily@MyHealth_MHND,Facebook&Google+. Originally published on LiveScience.

© 2012 LiveScience.com. All rights reserved.

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