UK scientists believe they have taken a significant step closer to a universal flu vaccine that would protect against all strains of seasonal flu and curb future pandemics.
The team, led by researchers at Imperial College London, reports the achievement in Nature Medicine.
Lead investigator Professor Ajit Lalvani, from the National Heart and Lung Institute at Imperial, says:
"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."
Flu virus always changing surface proteins
Current flu vaccines spur the immune system to make antibodies that recognize proteins on the surface of the virus. When these are spotted in an immunized person, the immune system mounts an attack against the virus.
The difficulty with this approach is that the flu virus is always changing its surface proteins, so different vaccines have to be developed every year as new strains emerge, leaving vaccine developers inevitably one step behind.
But the core of the flu virus is more stable and changes little as strains evolve, and researchers conducting lab experiments have suggested some immune cells may already protect against flu because they recognize unchanging proteins in the virus.
For instance, a group of US scientists recently took a step toward a universal flu vaccine with animal tests that suggested their vaccine caused the animals' immune system to make antibodies to those parts of the flu virus that do not change from strain to strain.
However, this latest study is the first to use a "natural experiment" in a real human pandemic, the 2009 swine flu pandemic.
Prof. Lalvani describes it as a unique opportunity to find out whether the immune system could recognize and protect us against new strains for which we lack antibodies.
He and his colleagues asked 342 volunteers to donate blood samples and undergo nasal swabs just as the pandemic got under way. They then tracked the volunteers by sending them email questionnaires about their health and any symptoms over the next two flu seasons. If any volunteers had flu symptoms, they took a nasal swab and sent it to the lab.
The team found the volunteers who caught the flu but had either no symptoms or only mild symptoms were those whose blood samples at the start of the 2009 swine flu pandemic had more CD8 T cells, a type of immune cell that kills viruses. Those who fell severely ill had fewer of these cells.
'Blueprint' for vaccine - make more CD8 T cells
The researchers believe a vaccine that spurs the immune system to make more CD8 T cells could protect against serious disease from all flu viruses, including those that cross into human populations from birds and pigs.
Prof. Lalvani says the findings provide a "blueprint" for developing a universal flu vaccine:
"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."
"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."
Such a vaccine could significantly limit seasonal flu and protect people against future pandemics, he adds.
It would be a different approach to conventional vaccines, which stimulate the immune system to develop antibodies in response to exposure to parts of a virus.
Researchers writing in PLOS Pathogens earlier this year, suggested a more effective route to a universal flu vaccine would be to combine T cell vaccines and antibody vaccines.
According to the World Health Organization (WHO), annual flu epidemics result in about three to five million cases of severe illness, and between one quarter and half a million deaths worldwide.