H7N9 bird flu has killed 44 people in China (Reuters)

Scientists have edged closer to a blueprint for a universal flu vaccine that would help to save over half-a-million lives every year.

A team of researchers at Imperial College London said they were getting close to developing a vaccine that would help to prevent flu viruses, including new strains that cross from animals to humans, such as the H7N9 bird flu that killed 44 people in China.

Study leader Ajit Lalvani said: "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."

The researcher asked volunteers to donate blood at the time of the 2009 swine flu pandemic, which killed 214 people in total.

The team used the outbreak as a "natural experiment" to find out why some people are able to resist severe illnesses.

Findings showed that people with good resistance had more CD8 T cells, a type of immune cell that kills viruses. In turn, people with fewer CD8 T cells became more seriously ill.

Swine Flu
Researchers took blood samples of people during the 2009 swine flu pandemic

They believe a vaccine that helped to stimulate the body to produce more of these cells would help prevent flu viruses. While this theory is not new, it is the first time scientists have been able to test it during a pandemic.

Lalvani said: "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.

"The 2009 pandemic provided a unique natural experiment to test whether T cells could recognise, and protect us against, new strains that we haven't encountered before and to which we lack antibodies.

"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.

"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."