"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," Professor Ajit Lalvani, chair of infectious diseases at the National Heart and Lung Institute at Imperial College London, said in a news release.
Lalvani and his colleagues began collecting blood from more than 340 volunteers during the 2009 H1N1, or "swine" flu pandemic. That flu strain killed at least 18,500 people around the globe, with researchers estimating up to 15 times more deaths that may have occurred due to related respiratory and cardiovascular illnesses.
Blood donors were asked in emails sent every three weeks to report any symptoms they experienced for a two-year period following the pandemic. If they had symptoms, they mailed in a nasal swab to a lab where scientists could confirm if the person indeed had the flu.
The scientists discovered that people who had a type of virus-killing immune cell called CD8 T cells were more likely to avoid serious illness during the deadly pandemic. That means a vaccine designed to increase CD8 T cell levels could be more effective at preventing flu viruses than conventional methods.
"The 2009 pandemic provided a unique natural experiment to test whether T cells could recognize, and protect us against, new strains that we haven't encountered before and to which we lack antibodies," said Lalvani.
The findings were published online Sept. 22 in Nature Medicine.
Current flu shots are developed based on certain strains in an effort to provide the best chances to reduce the risk of getting the flu or spreading it to others. A panel of infectious disease researchers and scientists select the strains that they think will be the most common for the year.
Once vaccinated, flu-fighting antibodies develop in about two weeks to stave off infection. That lag time is why experts recommend vaccination early in the fall before flu season peaks, which typically occurs later in the winter.
During last year's flu epidemic, the vaccine -- which protected against three strains -- was about 62 percent effective.
This year a "quadrivalent" vaccine was introduced to protect against four viral strains, available in shot or nose spray form. But, experts have said the new vaccine is not necessarily more effective because it protects against more strains.
The scientists behind the new research are hopeful that emerging flu strains that crossed over between humans and animals (like bird and swine flu strains) may too be stopped by a universal flu vaccine that increases CD8 levels. They point out unlike current shots that create antibodies to target specific strains, CD8 T cells target the core of the virus that never changes.
"We already know how to stimulate the immune system to make CD8 T cells by vaccination," said Lalvani. "Now that we know these T cells may protect, we can design a vaccine to prevent people getting symptoms and transmitting infection to others."
He told the BBC such a shot was about five years away.
The Centers for Disease Control and Prevention (CDC) says everyone older than six months should get a flu shot, especially high-risk groups including kids younger than 5, seniors 65 and over, pregnant women, patients with weakened immune systems and people who care for any of these people.