Researchers say adding a string of synthetic fat molecules to the seasonal flu vaccine boosted its effectiveness.
It is estimated that worldwide, flu infections are responsible for around 3-5 million cases of severe illness and 250,000-500,000 deaths every year.
Young infants, pregnant women, adults aged 65 and older and those with weakened immune systems are at greatest risk for flu-related complications.
The best protection against flu is the seasonal flu vaccine, which is developed every year based on a prediction of which viruses are likely to be circulating. However, such predictions are sometimes far from accurate; earlier this year, a report from the Centers for Disease Control and Prevention (CDC) revealed that the 2014-15 flu vaccine was only 23% effective across all age groups.
The low protection from last season's flu vaccine was put down to the emergence of influenza A H3N2 "drift variants" as the most predominant viruses, against which the vaccine had low effectiveness.
"The holy grail would be to develop a vaccine that cross-protects against different strains, which would be beneficial for the whole community, even if the prediction of circulating strains is wrong," says Brendon Chua, a research fellow at the University of Melbourne in Australia and coauthor of this latest study.
In addition, Chua and colleagues say such a vaccine would also be beneficial in the event that a strain of flu virus from another species evolves to infect humans; this has happened in the past with the H5N1 flu strain from birds and the H1N1 strain from pigs.
Activating both innate and adaptive immunity to combat flu
The team hypothesized that a more effective, cross-protective flu vaccine could be developed by using an adjuvant that activates a variety of antibody-independent immune responses.
The researchers had an adjuvant in mind to test - a synthetic lipopeptide, which consists of a string of fat molecules that simulate a lipopeptide found on the outer membrane of a pathogen. They explain that this adjuvant activates both innate and adaptive immune responses.
The innate immune response is the body's first-wave, short-term defense against pathogens to help prevent cells from becoming infected, while the adaptive immune response is the longer-term defense in which immune cells learn to "remember" pathogens they have previously encountered to launch a more effective attack.
"Harnessing both types of immunity would provide protection in that period during an outbreak when no [new] vaccine is available," notes Chua.
Increased protection with adjuvanted vaccine
For their study, published in the journal mBio, the researchers gave a group of mice a low dose of an inactivated influenza A vaccine in which the synthetic lipopeptide had been added, while another group was given an inactivated influenza A vaccine without the adjuvant. Three days later, the mice were exposed to the flu strain that was included in the vaccine, as well as an "unmatched" strain that was not.
- The CDC recommend that every person aged 6 months and older receives an annual flu vaccine
- More than 350 million doses of the 2015-16 flu vaccine have been distributed in the US so far
- It takes around 2 weeks after flu vaccination for protection against the virus to begin.
The team found the mice that received the adjuvanted vaccine demonstrated much better protection against both flu strains, compared with mice that received the standard flu vaccine, and they even survived a normally lethal dose of the flu virus.
What is more, the researchers found the mice that received a low dose of the adjuvanted vaccine produced around 600 times more neutralizing antibodies than mice that received a similar dose of the standard vaccine, and the adjuvanted vaccine also stimulated a higher number of T cells responsible for clearing flu infection in the lungs.
"The culmination of all these responses is that it reduces the ability of the virus to infect cells, reproduce and spread," says Chua.
The researchers then infected mice with flu virus strains - only one of which was included in the vaccine - 35 days after giving them either the low-dose adjuvanted vaccine or standard vaccine.
The mice that received the adjuvanted vaccine showed significant protection against both strains, while mice that received the standard vaccine demonstrated low protection against both strains, according to the results.
What is more, the researchers found that the adjuvanted vaccine reduced transmission of the unmatched flu virus from vaccinated to unvaccinated mice that had lived in the same cage for 2 days - a key finding. Chua explains:
"The biggest advantage is that this approach doesn't rely on getting a match between the strains used in the vaccine and circulating virus - you can still get some protective effect at the population level."
The team believes their findings suggest that adding a simple component to the seasonal flu vaccine may not only lead to more effective protection against the flu virus, but it could also offer community protection against a new flu strain in the early stages of an outbreak.
In August, Medical News Today reported on two new studies in which researchers revealed the development of new vaccine candidates that may lead to a universal flu vaccine.