Many humans may already have some immunity to H5N1 bird flu, raising hopes that the virus could pose less of a threat than previously thought, new research suggests.  Tests on hospital workers in the Netherlands found that those who had either been previously infected with seasonal flu or received the flu vaccine had special antibodies in their blood that help fight H5N1 – the strain scientists have warned could trigger the next global pandemic .  While the antibodies, known as cross-reactive antibodies, do not stop the virus from entering the body, they do signal other cells to start fighting off the infection, strengthening the response.  This so-called alert mechanism could mean that an H5N1 infection results in less severe illness, the researchers from Rotterdam’s Erasmus University Medical Centre found.…