Influenza – or the flu – is a highly infectious, viral illness. The flu shot is a seasonal vaccine that only protects against a certain type of the flu virus. This means people have to receive the flu shot every year as the virus that causes the flu changes annually. Finding a vaccine that can protect people against multiple variants of the virus has been quite difficult, but two different studies report an important breakthrough.

Researchers have developed a nanoparticle vaccine that could be an important step to developing long-lasting protection. The two proof-of-concept studies are detailed in the journal Science and Nature Medicine.

The nanoparticle vaccine developed for one subtype of the flu virus was found to completely protect mice with different types of the influenza virus and also partially protected ferrets against a different subtype. Researchers from the National Institutes of Health suggest they’ve cleared an important hurdle for so-called broad protection – which is when the vaccine can protect against multiple variants of the virus.

“This study shows that we’re moving in the right direction for a universal flu vaccine,” Ian Wilson, a professor of structural biology at The Scripps Research Institute, said in a statement.

Researchers targeted a protein on the surface of the flu, called hemagglutinin (HA). This protein is found in all types of the flu virus. As BBC News explains, the flu virus is like “a ball with lots of lollipops on stems sticking out. The lollipops change year to year, but the stems remain the same. It is the stems that scientists are now focusing on as a target for a universal flu jab.”

HA is important as it provides the key viral “machinery” that allows the virus to enter cells. Though it's less accessible than the head region, the stem has the added advantage of undergoing little to no mutations. “If the body can make an immune response against the HA stem, it’s difficult for the virus to escape,” Wilson explained.

The vaccine is a particularly promising candidate that could teach the body to make powerful antibodies and mount an immune response to a variety of flu strains. While the ultimate goal is to create a vaccine with broad, long-lasting protection, researchers need to do further research on humans.

Sarah Gilbert, a professor of vaccinology at University of Oxford, told BBC News: “This is an exciting development, but the new vaccines now need to be tested in clinical trials to see how well they work in humans.”

“This will be the next stage of research, which will take several years. So we are still some way from having better flu vaccines for humans,” she added.