Could some people have a little immunity to the H5N1 bird flu virus? One study in mice published on Monday suggests it is, in theory, possible.

They found mice inoculated with a human virus known as H1N1 were less likely to die when they were infected with a little bit of H5N1 -- although this protection went away after a bigger dose of H5N1.

The finding suggests it is possible that some people previously infected with or vaccinated against flu may have a slight protection from H5N1, the researchers wrote in the Public Library of Science journal PLoS Medicine.

"It is weak protection," said Richard Webby at St. Jude Children´s Research Hospital in Memphis, Tennessee. "It is not protection from infection -- it is protection from death."

Researchers have wondered if people who have been infected with one strain of influenza might have partial immunity to another strain. They know no one has complete immunity, because flu can infect the same person over and over again.

But maybe there is just enough there to keep the new infection from being deadly.

"The idea has been thrown around for a while and we just decided to actually test it," Webby said in a telephone interview.

The H5N1 avian influenza virus has infected 272 people since 2003 and killed 166 of them. Researchers worry it could mutate into a form that would spread around the world and kill millions of people.

No human has immunity to H5N1 so far as any anyone knows.

It is possible, Webby´s team wrote, that some people have been infected with H5N1 and never became seriously ill because they have some natural immunity. So far, the virus has killed 60 percent of those known to have been infected.

This could explain why 90 percent of H5N1 victims have been under 40, the researchers said. Older people may have some immunity from earlier influenza infection, although there is no proof of this and there are other explanations for why younger people are more likely to be infected or diagnosed.

CONSTANT MUTATIONS

There are hundreds of different influenza strains, known by their "H" and "N" designations.

The H refers to hemagglutinin, the protein that the flu virus uses to get into cells, and the N refers to neuraminidase, which it uses to get back out of infected cells and spread to others.

What is most different about H5N1 flu is the H5 part. People have been catching H1N1 flu since at least 1918, when the worst-ever-documented influenza pandemic killed at least 50 million people globally.

A much less dangerous descendant of H1N1 circulates today, and is part of the annual seasonal flu vaccine.

So the question was whether having immunity to N1, either from natural infection or the vaccine, might help protect people from the worst effects of H5N1.

"Antibodies to the human version of the N1 do cross-react to some extent with the H5N1," Webby said.

The match is not perfect, because influenza viruses mutate constantly and the N1 in human seasonal flu looks substantially different from the N1 in H5N1 avian flu.

Webby´s team vaccinated mice against human N1 and then exposed them to various flu viruses.

Five out of 10 mice infected with a small dose of the full H5N1 virus lived, although all the mice infected with a big dose of H5N1 died.

They also tested human blood samples against two strains of H5N1 from human victims and found eight out of 38 had antibodies that reacted to one strain, and nine of 38 responded to the other.

Vaccines against seasonal flu focus on the "H" part of the virus, because they are meant to block infection completely. It is not clear how much "N" component current vaccines have, Webby said.