A universal flu vaccine that would not have to change every year may become a reality thanks to the swine flu pandemic in 2009.

Scientists say they used the pandemic as a “natural experiment” to discover how the body’s immune system builds resistance to the flu. Their research, published in the journal Nature Medicine, showed how certain immune cells helped some avoid the severe illness.

"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,” said study leader Professor Ajit Lalvani, from the National Heart and Lung Institute at Imperial College London, in a statement. “This provides the blueprint for developing a universal flu vaccine.”

The team used donated blood from more than 300 staff and students at Imperial College London at the start of the swine flu pandemic in 2009 and monitored their symptoms. Those who contracted the flu had fewer CD8 T cells in their blood, while those who remained healthy had more of these virus-killing immune cells.

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

A new vaccine that would stimulate the body to create more of these cells may be effective in protecting against all strains of the serious disease.

"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," Lalvani said.

Knowing that the CD8 T cells may protect individuals from contracting the virus, Lalvani said, puts scientists one step closer to developing a groundbreaking vaccine. "We already know how to stimulate the immune system to make CD8 T cells by vaccination. Now that we know these T cells may protect, we can design a vaccine to prevent people getting symptoms and transmitting infection to others. This could curb seasonal flu annually and protect people against future pandemics."

The swine flu outbreak killed an estimated 284,500 people in 2009. Caused by the H1N1 influenza virus, the swine flu contained bits of bird, swine and human flu viruses, a combination that had never been detected before.

According to the U.S. Centers for Disease Control, one of the major reasons behind the staggering number of deaths was lack of accessible health care.  

"This pandemic really did take an enormous toll," Dr. Fatimah Dawood of the CDC told Reuters. "Our results also suggest how best to deploy resources. If a vaccine were to become available, we need to make sure it reached the areas where the death toll is likely to be highest."