CHICAGO (Reuters) - A study of antibodies from people infected with H1N1 swine flu adds proof that scientists are closing in on a “universal” flu shot that could neutralize many types of flu strains, including H1N1 swine flu and H5N1 bird flu, U.S. researchers said on Monday.

They said people who were infected in the H1N1 pandemic developed an unusual immune response, making antibodies that could protect them from all the seasonal H1N1 flu strains from the last decade, the deadly “Spanish flu” strain from 1918 and even a strain of the H5N1 avian flu.

“It says that a universal influenza vaccine is really possible,” said Patrick Wilson of the University of Chicago, who worked on the paper published in the Journal of Experimental Medicine.

Many teams are working on a “universal” flu shot that could protect people from all flu strains for decades or even life.

U.S. officials say an effective universal flu vaccine would have enormous ramifications for the control of influenza, which kills anywhere from 3,300 to 49,000 people in the United States each year.

Wilson’s team started making the antibodies in 2009 from nine people who had been infected in the first wave of the H1N1 swine flu pandemic before an H1N1 vaccine had been produced. The hope was to develop a way to protect healthcare personnel.

Working with researchers from Emory University School of Medicine, the team produced 86 antibodies that reacted with the H1N1 virus, and tested them on different flu strains.

Of these, five were cross-protective, meaning they could interfere with many strains of flu including the 1918 “Spanish flu” and a strain of H5N1 or avian flu.

Tests of these antibodies in mice showed they were fully protected from an otherwise lethal dose of flu.

And some of these cross-protective antibodies were similar in structure to those discovered by other teams as having potential for a universal flu vaccine.

“It demonstrates how to make a single vaccine that could potentially provide permanent immunity to all influenza,” Wilson said in a telephone interview.

“LOLLIPOP STICK”

Flu vaccines and drugs focus on proteins found on the surface of the flu virus called hemagglutinin and neuraminidase, which give influenza A viruses their names, as in H5N1 or H1N1.

A woman receives a H1N1 influenza vaccine shot from a medical staff at a hospital in Nonthaburi province, on the outskirts of Bangkok January 11, 2010. REUTERS/Chaiwat Subprasom

Hemagglutinin is a lollipop-shaped structure with a big, round head. This head is so large it attracts most of the immune system antibodies, but it mutates readily.

Two years ago, researchers working for Crucell NV and a separate team found that antibodies that attach to the “stick” or stalk part of the hemagglutinin lollipop mutate much less -- providing a perfect target for a vaccine that could neutralize a range of different flu viruses.

“Previously, this type of broadly protective, stalk-reactive antibody was thought to be very rare,” Jens Wrammert of Emory said in a statement. But in the H1N1 patients, he said, they were “surprisingly abundant.”

That may be because the H1N1 virus was so different from other flu strains that the immune system made antibodies for the only parts of the virus it recognized -- this “stick” or stalk region that is common to many flu strains.

Wilson said the study proves it is possible to get the immune system to make these antibodies if it has the right stimulation. The team is working with an unnamed biotechnology company to develop a vaccine or drugs based on this notion.

And a team at the National Institutes of Health is testing a two-step vaccine that uses DNA from stalk-reactive antibodies to “prime” the immune system, followed by a regular flu shot.

A study in July showed this two-step approach protected mice and ferrets against flu strains from 1934 through 2007. This vaccine is now being tested in people.