A universal vaccine that offers protection against multiple flu strains could soon be within reach of scientists searching for the "Holy Grail" of immunology.
Two teams of researchers have reported major progress towards creating a vaccine that could win the arms race with influenza, which continually mutates to dodge the immune system.
Both focused on the "stem" of a key component of the virus that remains unchanged in different strains, but is not easily targeted by the body's defences.
Flu jabs are currently designed around the "head" of the haemagglutinin (HA) molecule that alters from year-to-year, making it necessary to keep up-dating the seasonal vaccine.
As well as being reliably effective against seasonal flu, a universal vaccine would also prevent illness caused by other viral sub-types, such as those that normally infect birds and pigs.
One group combined the HA stem with part of an iron-storing bacterial protein to create a star-shaped nanoparticle that triggered an immune response in mice and ferrets.
Most of the animals infected with a lethal dose of H5N1 bird flu survived after receiving the "nano-vaccine", the scientists reported in the journal Nature Medicine.
The US researchers, led by Dr Barney Graham and Dr Gary Nabel, from the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland, wrote: "Our results establish a proof of concept - that elicitation of non-neutralising antibodies by an HA stem-only nanoparticle vaccine can provide broad protection against severe disease."
A separate team, led by Dr Antonietta Impagliazzo, from the Crucell Vaccine Institute in Leiden, the Netherlands, described in the journal Science how they decapitated HA and modified the remaining stem to help it bind to immune system antibodies.
In mice, one configuration of the vaccine closely mimicking the complex structure of the full HA stem provided complete protection against both H5N1 and H1N1 (swine flu) strains.
A strong immune response was also seen in monkeys treated with the vaccine, whose swine flu fevers were significantly reduced.
Commenting on the findings, British vaccinology expert Professor Sarah Gilbert, from Oxford University, said: "We've known for some time that there is a region of HA, the stem, that does not change and is present on all flu A viruses, and if we can use only that part in the vaccine we could raise immunity to many different viruses at the same time, but it has been technically challenging to make a vaccine that works in that way.
"This is an exciting development, but the new vaccines now need to be tested in clinical trials to see how well they work in humans. This will be the next stage of research, which will take several years. So we are still some way from having better flu vaccines for humans."