London — Scientists in Britain have used gene-editing techniques to stop bird flu spreading in chicken cells grown in a lab — a key step towards making genetically-altered chickens that could halt a human flu pandemic.

Bird flu viruses currently spread swiftly in wild birds and poultry, and can at times jump into humans. Global health and infectious-disease specialists cite as one of their greatest concerns the threat of a human flu pandemic caused by a bird-flu strain that makes such a jump and mutates into a deadly and airborne form that can pass easily between people...