Partial Immunity to H5N1 Flu Possible in Some

MEMPHIS, Feb. 13 -- Some people may have at least a partial immunity to the highly pathogenic H5N1 avian influenza virus, according to researchers here.

If a flu pandemic develops on the basis of a mutation in the H5N1 virus, such partial immunity might limit the virulence of the disease, although it would probably not prevent infection, said Matthew Sandbulte, Ph.D., of St. Jude Children's Research Hospital.

This conclusion was based mainly on experiments showing that mice immunized against the neuraminidase enzymes of the currently circulating H1N1 human flu develop antibodies against the similar neuraminidase of the avian flu, Dr. Sandbulte and colleagues reported online in the journal PLoS Medicine.

But the researchers were also able to show that some humans - about 20% of a small sample -- already have antibodies able to inhibit the activity of the avian neuraminidase.

The finding "underscores a possible benefit of seasonal influenza vaccination," Dr. Sandbulte and colleagues said - as well as protection against the circulating flu, it might offer some protection against a pandemic strain.

Current vaccines target the most abundant viral surface antigen, hemagglutinin and have varying amounts of neuraminidase. But the vaccines produce antibodies to both antigens, the researchers said, and neuraminidase antibodies can inhibit viral replication and decrease disease severity.

The clearest example, the researchers said, was the pandemic strain of 1968, which was an H3N2 virus. Although the pandemic caused high mortality and morbidity, it was somewhat limited because many people had antibodies to the neuraminidase in previously circulating H2N2 strains, which was identical to the pandemic strain.

Using DNA vaccines, the researcher immunized mice with the neuraminidase from a human H1N1 virus, and saw that the vaccination produced neuraminidase antibodies.

When the mice were challenged with another H1N1 strain, they sickened and lost weight but did not die and later recovered. In contrast, all of a group of control mice died. The difference between the groups was significant at P

They also noted that it's impossible to predict which flu strain will cross the species barrier to cause a pandemic, even though H5N1 is the current favorite.

Because of that, it's "unlikely" that vaccinations aimed at any specific neuraminidase would match an emerging pandemic virus, they said.