HERE are two pieces of good news about bird flu: first, combining genes from H5N1 with those from a common human flu virus doesn鈥檛 seem to have instant pandemic potential. Second, an H5N1 vaccine stimulates a strong immune response so, if the worst happens, we may be able to protect ourselves.
H5N1 meets two of three requirements for a pandemic strain: it is highly pathogenic and unfamiliar to most people鈥檚 immune systems. If it also acquired the ability to transmit easily from person to person, H5N1 would threaten millions of lives. One way this could happen is if the virus swapped genes with human flu.
A team at the US Centers for Disease Control (CDC) in Atlanta, Georgia, engineered flu viruses that contained genes for the external 鈥渃oat鈥� proteins of H5N1 and the internal proteins of H3N2, the most common circulating variety of human flu. They then inoculated ferrets with these 鈥渞eassortant鈥� viruses.
Advertisement
When ferrets are in adjacent cages, H3N2 passes readily from one cage to the next. Neither H5N1, nor the new viruses, could be transmitted in this way, and the engineered strains were much less pathogenic than H5N1 (Proceedings of the National Academy of Sciences, DOI: 10.173/pnas.0605134103).
This suggests it may take more than a simple exchange of genes to turn H5N1 into a global killer, although the team has yet to explore other possible reassortants.
However, the virus that caused the 1918 pandemic, which killed more than 40 million people, probably evolved by the mutation of a bird flu strain rather than recombination with a human virus.
Had the CDC experiments revealed the engineered viruses to be extremely dangerous, it would have increased pressure to churn out H5N1 vaccines. Last week, GlaxoSmithKline announced that low doses of its vaccine, based on inactivated H5N1, stimulated a strong immune response in 80 per cent of volunteers, a much better result than in previous tests. Such a vaccine may not protect against the virus as it mutates and evolves, though. 鈥淲e are far from out of the woods with H5N1,鈥� says CDC director Julie Gerberding.
