influenza

While current flu shots need to be updated each season, scientists are finding new ways to make vaccine that could last much longer and cover more strains. It's a miserable disease – and, unfortunately, there's no guarantee that if you catch it one year, you go through the same ordeal the following year. Influenza is a wily virus, constantly shapeshifting to get around humans' immune defences. "That's why you have to get a flu shot every year," says Nicholas Heaton, a professor at the Duke University School of Medicine in North Carolina, US. Seasonal flu vaccines prevent many deaths and serious illnesses each year, but they are imperfect. Their effectiveness typically tops out around 60% and can dip well below that in years when the vaccine's formula isn't a good match for the virus that is actually spreading among humans. But what if you didn't have to get a new shot every year? Heaton's lab, and others around the world, are trying to answer that tantalising question. They're developing so-called "universal" flu vaccines, which aim to offer better, broader and more durable protection than current seasonal vaccines. The idea is to "cover more strains or make the shot last longer", Heaton says. "Or, hopefully, both." There are currently about a dozen such vaccine candidates moving through the clinical trial process and many others still in earlier stages of development, according to an initiative that tracks next-generation influenza vaccine development . The term "the flu" is a bit misleading. Influenza is not a single entity, but many different viruses circulating between people and animals. And these viruses are constantly evolving , allowing them to stay one step ahead of human immune systems. "Flu varies a lot," Ostrowsky says. "It's just a constantly moving target." Some of the shapeshifting components are proteins on the surface of each virus called haemagglutinin and neuraminidase. When you're exposed to influenza, your immune system recognises these proteins and responds in kind, fighting off infection by creating antibodies that bind to them. But the viral proteins are hardly sitting ducks. There are 18 varieties of haemagglutinin and 11 varieties of neuraminidase , which can mix and match to create different flu subtypes, such as H1N1 and H3N2. The viral genes that create these proteins also rack up mutations, resulting in new forms of the virus, splitting into clades and sub-clades. A mutation here or there may not be enough to outsmart the immune system. But over time, the changes accumulate enough that the body's immune defences are rendered out of date, forcing them to play catch-up. That's why it's so difficult to formulate the flu shot each year. Public health officials and vaccine makers are, in effect, trying to predict the future, making their best guesses about how the virus will change and which specific strains will circulate in the season ahead. To do so, the World Health Organization (WHO) convenes an international group of experts to pore over reams of data on where influenza is spreading and how it's evolving. Last time they printed all this data, they racked up a 10cm thick stack of double-sided papers, says Wenqing Zhang, from the WHO's department of Epidemic and Pandemic Threats Management. Each February, the WHO's expert group meets to create recommendations for shots used in the Northern Hemisphere, where flu season typically starts around October. In September, they do the same for the Southern Hemisphere, where flu season typically starts around April. But a lot can change after these predictions are made. In the 2025-2026 season, for instance, flu outbreaks have been occurring around the world caused by H3N2 subclade K influenza viruses, which weren't even on the WHO's radar in February 2025. "At that time, there was no clade K virus yet," Zhang says, so there wasn't a recommendation to tailor vaccines accordingly. Despite the surprise, preliminary data out of the UK suggest this ye