Catch the flu to avoid dying from Bird flu?

A recent study suggests that immunity to ordinary flu may confer some degree of resistance to the H5N1 bird flu virus. The study involved innoculating mice with the human virus H5N1.
The study found that these mice were less likely to die when they were then infected with a small amount of H5N1 - although protection disappeared after a greater dose of H5N1. 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. This suggests it is possible that people previously infected with or vaccinated against flu may have soem degree of protection from H5N1.

Richard Webby (St. Jude Children's Research Hospital, Memphis, Tennesse) writing in the Public Library of Science journal PLoS Medicine said "It is weak protection ...It is not protection from infection -- it is protection from death."

There has been speculation about the possibility that people previously infected with one strain of influenza might show partial immunity to another strain. It has always been clear that this could not confer complete immunity - because flu can infect the same person over and over again.

But there may be enough protection to stop a new infection from mutated H5N1 from being deadly to those individuals.

Webby said that it was this possibility that they wanted to test. No human has immunity to H5N1 yet as far as is known. H5N1 avian influenza virus has infected over 270 people since 2003 and over 160. The fear is that it may mutate into a form which will readily spread from human to human and kill millions of people. One of the characteristics of the Flu virus in all its forms is its ability to mutate.

Webby's team speculate that some people may have been infected with H5N1 and not 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 may explain why 90% of H5N1 victims are aged under 40. Older people may have a degree of immunity from earlier influenza infection. There is no clear proof of this yet and it is thought there are other explanations for younger people being more likely to be infected or diagnosed.

Influenza strains are referred to by their "H" and "N" designations. The H being the type of Hemagglutinin, the protein that the flu virus uses to get into cells, and the N is Neuraminidase, which the virus uses to get back out of infected cells and spread to others.

People have been catching H1N1 flu since at least 1918, when the worst-ever-documented influenza pandemic killed at least 50 million people globally. The type of flu which normally circualtes today is a much less dangerous descendant of H1N1 circulates and this is used the annual seasonal flu vaccine. The H5N1 flu virus differs mainly in the H5 part.

The issue therefore is whether immunity to N1, either from natural infection or the vaccine, might provide some protection 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. Because influenza viruses mutate constantly the match is not perfect and the N1 in human seasonal flu looks substantially different from the N1 in H5N1 avian flu. 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.

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.