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Flu viruses are composed of eight segments, including the haemagglutinin protein, or “H” part of the virus, and neuraminidase, or the “N” part.

When an animal is infected by two different flu viruses, the genetic material of the two can switch or recombine to form a new version — which is what probably happened here: While the haemagglutinin — the part of the virus that allows it to attach to a host — remained the same, a new neuraminidase — the part that helps release the virus from infected cells — was substituted in.

Anna Blakney, an RNA bioengineer at the University of British Columbia, told the journal Nature that there is no guarantee mRNA will be an effective vehicle for transporting haemagglutinin glycoproteins, the protein that flu vaccines use to fight the different bugs.

One way this could happen is if the gene for haemagglutinin, an important protein on the surface of the virus, underwent an amino acid–swapping mutation that replaced a particular glycine, more often seen in bird flu viruses, with an aspartic acid, which is more characteristic of human viruses.

On these viruses, the activation site is found on a protein called haemagglutinin, not on the spike protein.