Since the 2009 swine flu pandemic, scientists have been working to determine why some people were resistant to the severe illness in order to develop a stronger vaccine, and that may be coming soon.

According to a press release, those who were resistant to the virus had a higher count of CD8 T cells, a virus-killing immunity cell. Imperial College London (ICL) scientists began collecting blood samples as the 2009 pandemic began and asked those donors to report on their next two flu seasons.

As a result of their findings, published in the journal Nature Medicine, the researchers believe the answer is a vaccine that stimulates the body into producing these cells. This vaccine should block all types of flu strains, including those found in birds and pigs.

"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," lead author of the study Ajit Lalvani, of ICL's National Heart and Lung Institute.

Current flu vaccines produce antibodies for the immune system that work to prevent infection from the most common strains. These vaccines have to be changed every year as the virus evolves to be resistant against the previous vaccines.

In autumn of 2009, Lalvani had to recruit participants for his team's study swiftly. The researchers rounded up 324 ICL staff and students to take part and give blood samples and nasal swabs. Every three weeks afterward, they were sent an email with a survey on their health and, if they had flu symptoms, they were asked to submit a nasal swab.

They found that the participants who severely came down with the flu lacked the CD8 T cells and those who experienced mild or no symptoms had more.

"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," Lalvani said. "This provides the blueprint for developing a universal flu vaccine."

Lalvani told BBC News he thinks the vaccine is just five years away.

"In truth, in this case it is about five years [away from a vaccine]," he said. "We have the know-how, we know what needs to be in the vaccine and we can just get on and do it."