* Novartis, Novavax H7N9 vaccines give good immune response

* Companies using new technologies to accelerate production

* H7N9 flu emerged as human threat in China last winter (Updates with details on Novartis vaccine)

By Sharon Begley and Ben Hirschler

NEW YORK/LONDON, Nov 14 (Reuters) - The first human tests of experimental vaccines against a deadly strain of avian flu, using novel technology that could produce millions of doses very quickly, has produced protective antibodies in the vast majority of recipients.

Encouraging results in early-stage trials were announced for separate vaccines from Swiss drugmaker Novartis and Novavax, a biotech company based in Rockville, Maryland.

Details of the Novavax vaccine were published online in the New England Journal of Medicine late on Wednesday, while Novartis disclosed its positive findings on Thursday.

"These are very preliminary results, but it appears for the first time that we may have a vaccine that would work against an outbreak" of avian flu, said Robin Robinson, director of the Biomedical Advanced Research and Development Authority, or BARDA, the federal agency in charge of developing countermeasures against public health emergencies.

Because other candidate vaccines against avian flu have failed, "this is a very important milestone", he said. "We have a promising vaccine where before we had none."

The H7N9 strain of avian flu emerged in China last winter. There had been 45 deaths from 137 confirmed cases this year as of late October, according to the World Health Organisation. Cases and deaths, often from severe pneumonia, both peaked in March and April.

But public health experts fear the virus could come storming back this flu season. After no reported cases of H7N9 in China in August or September, there have been four since early October.

A mortality rate of one-third suggests the virus is highly lethal.

The WHO says there is currently "no indication" the virus can be transmitted from person to person, and so cannot become a pandemic. But flu strains can quickly undergo genetic changes that make them transmissible between people.

In the Novartis clinical trial, 400 healthy adult volunteers received two doses of either a dummy injection (placebo) or different formulations of the experimental vaccine - either with or without an adjuvant, a chemical compound that turbo-charges the immune system.

Of those given the adjuvanted vaccine, 85 percent had a protective immune response, against only 6 percent for those getting the vaccine without the adjuvant.

The Novavax study involved 284 volunteers who also received vaccine formulations with or without an adjuvant.

At the heart of the vaccines are two proteins, dubbed H7 and N9, that stick out from the virus and give it its name. The Novavax vaccine triggered production of antibodies against the "H" protein in 81 percent of the volunteers who received the vaccine with the high level of adjuvant, and antibodies against the "N" in more than 90 percent.

Antibodies are molecules produced by the immune system that attack invaders.

The studies did not expose volunteers to the virus, which is considered unethical, to see if the antibody levels warded off infection. "But these antibody levels are very likely to be protective," said Dr Louis Fries, Novavax's vice president for clinical and medical affairs, who led the Novavax study.

Andrin Oswald, head of Novartis Vaccines, said he believed the Novartis vaccine was "able to offer a protective solution for a potentially deadly pandemic virus".

FAST MANUFACTURING

Just as important as the new vaccines' apparent efficacy is how quickly they can be produced, thanks to eliminating the need to use chicken eggs as most flu vaccine production does.

Fast manufacturing is important because a pandemic flu strain can emerge with little warning. An initial outbreak of a new flu strain is often followed by a more severe and widespread outbreak the following flu season.

Novartis has been an early pioneer of full-scale cell-culture manufacturing, an alternative to egg-based flu vaccine technology that can accelerate production significantly.

Novavax uses a process that takes the virus's genetic sequence and produces what it calls a "virus-like particle vaccine".

Virus-like particles, or VLPs, contain three proteins that trigger the production of antibodies and other immune responses to thwart the virus. The H and N proteins generate antibodies that keep the virus from replicating and infecting cells, while a third protein stimulates killer T cells to slay any cells already infected.

Using Novavax's technique, said BARDA's Robinson, computer models suggest manufacturers could produce the first doses of H7N9 vaccine within 12 weeks of the emergence of a pandemic, 50 million doses within four months, and hundreds of millions of doses within six months.

Novavax has a three-year, $97 million contract with BARDA to develop recombinant influenza vaccines and manufacturing capabilities sufficient to produce enough vaccine for a pandemic. It is planning a second trial of the VLP vaccine in early 2014 to nail down the minimum effective dose, said Fries.

Novartis has also received funding from BARDA for its research.

Other candidate H7N9 vaccines are being developed, said Robinson, with results expected in four to six months. If they also work, public health authorities around the world would have multiple vaccines to choose from should an avian flu pandemic emerge. (Editing by Peter Cooney and Pravin Char)