The dreamlike goal of having universal, one-off flu jabs just got a little bit closer, thanks to the discovery that certain immune cells can fight off all three strains of influenza – perhaps permanently, or at least for several years.

At the moment, the various types of influenza - strains A, B, and C - are put into the annual vaccine at different rates every year to stay ahead of the mutated versions of the virus. If our immune system could fight all of them, we wouldn't need to keep vaccinating so often.

The potential for particular immune cells to take care of all three flu strains was spotted in an earlier analysis of people exposed to the H7N9 (bird flu) virus in 2013. Those who had a strong response from CD8+ T cells were much more likely to recover.

These CD8+ T cells are often known as 'killer cells' because of the way they fight incoming threats – like a security force guarding the gates of our bodies.

"Our team has been fascinated by the killer cells for a long time," says lead researcher Katherine Kedzierska, from the University of Melbourne in Australia. "So our next step was to discover how their protective mechanism worked, and if it had potential for a flu vaccine."

This is where the new research comes in. Mass spectrometry analysis was used to sift through 67,000 viral sequences, looking for specific peptides or chemical bonds common among all three flu strains in humans.

Particular combinations known as epitopes can act as flags to CT8+ T cells, telling them a virus has arrived and initiating the orders to kill it.

"We identified the parts of the virus that are shared across all flu strains, and sub-strains capable of infecting humans," says one of the team, Marios Koutsakos from the University of Melbourne.

In tests on mice, the team then used these parts of the virus to immunise the animals against the flu, and it worked – infection and inflammation levels were "remarkably reduced" the researchers say.

There's still a long way to go before we have an all-in-one flu jab that's ready to be used, though. According to the team's estimates, roughly 54 percent of the world's population have the right type of CT8+ T cells in their bodies that can initiate this protective immune response.

That said, this is a key step forward in working out how we might develop better tools for fighting influenza – by getting our killer cells to see off all incoming strains, instead of the annual dance around the strains we think are most likely to hit.

Strain A is usually associated with flu pandemics (large scale spread over multiple countries), while strains A and B are associated with annual epidemics (notable rises in a more limited geographical area). Strain C is less common but can cause serious illness in children.

Importantly, these strains mutate frequently, quickly changing from sub-strain to sub-strain, meaning that even getting a jab every year isn't always effective.

A jab to protect us from all of the strains at once could prevent thousands of deaths a year. And if this option doesn't work out, we've got others to explore: last year scientists harnessed antibodies found in camelids – camels, alpacas, and llamas – to protect against strain A and strain B in mice.

"This work highlights the underlying power and versatility of the mass spectrometry approach, and we're excited about the future potential of these epitopes in the development of universal vaccines," says one of the researchers, Anthony Purcell from the Monash Biomedicine Discovery Institute in Australia.

The research has been published in Nature Immunology.