A group of scientists say they have found a clue that could lead to developing an influenza vaccine that will not require changing every year with each new strain.

Researchers used the 2009 swine flu pandemic as a sort of natural experiment, gleaning from it a way of understanding why some people resist influenza better.

The research was published in the journal Nature Medicine today.

"New strains of flu are continuously emerging, some of which are deadly, and so the Holy Grail is to create a universal vaccine that would be effective against all strains of flu,” Prof. Ajit Lalvani from the National Heart and Lung Institute at Imperial College London, who led the study, said in a release.

Researchers at Imperial College in the U.K. collected blood samples from 342 volunteers (both staff and students at Imperial) during the swine flu outbreak and collected data on all symptoms experienced over the next two flu seasons.

People who managed to avoid severe illness as a result of the flu strain were found to also have more virus-killing immune cells (CD8 T cells) in their blood at the onset of the pandemic, according to the research.

Based on that discovery, researchers assumed a vaccine that could stimulate production of these cells might be effective in preventing more flu viruses.

"The immune system produces these CD8 T cells in response to usual seasonal flu. Unlike antibodies, they target the core of the virus, which doesn't change, even in new pandemic strains," Lavlani said.

"Our findings suggest that by making the body produce more of this specific type of CD8 T cell, you can protect people against symptomatic illness. This provides the blueprint for developing a universal flu vaccine.”

The flu vaccines that are already out there make the immune system produce antibodies that recognize structures on the surface of the virus and prevent infection with the most common strains of influenza. Because of that, they have to be changed and updated each year as popular strains change and evolve.

The aforementioned T cells were previously thought to protect against the flu, but were only recently tested with regards to humans during a pandemic.