It isn’t necessarily obvious what part of a flu virus any potential mRNA vaccine should aim for. “You need to make sure you’re targeting the right part of the virus,” says Scher. With Covid-19, the prominent spike protein fit the bill. But influenza viruses are arguably more complicated and mutate more quickly, meaning that if you pick the wrong protein, your jab could prove less effective than hoped. The flip side, Scher suggests, is that mRNA vaccines could make it possible to target multiple proteins or parts of proteins on the same virus—a multipronged strategy.

And while they’re tricky to develop, the speed with which mRNA vaccines can be produced could be hugely beneficial. Traditionally, flu vaccines contain inactivated viruses that are grown in hens’ eggs. This works reasonably well, but it takes a long time to make such jabs, which means health authorities have to publish their predictions about which strains of flu will be circulating during the upcoming winter well in advance. If you could manufacture vaccines more quickly, you could make more accurate predictions nearer to flu season.

Not only that, researchers hope that a single mRNA shot could one day target 20 or more strains of flu at once, relieving the need for some of this guesswork. Scher’s colleagues are working on such a “universal” flu vaccine.

With clinical trials ongoing, it’s still early days. Sheena Cruickshank, an immunologist at the University of Manchester, has watched reports about emerging mRNA flu jabs with interest but says that questions remain. “We don’t yet know how long-lasting the immunity they produce is,” she says.

Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, concurs, though he notes that all flu jabs, regardless of how they are made, have a waning immunity problem—your protection could decline by around 10 percent every month following injection.

A concern specific to mRNA vaccines is that they tend to cost more than traditional flu vaccines and must be kept refrigerated, which may make them difficult to roll out in areas with poor infrastructure. Researchers are also concerned that they may meet with more vaccine hesitancy. “The mRNA vaccine platform, per se, is probably the one that seems to get the most misinformation,” notes Cruickshank. “That could be a disadvantage.”

A new wave of mRNA flu vaccines could be particularly impactful for older patients, says Jenna Bartley, an assistant professor at UConn Health, a health research center and hospital. Older people are among the most at risk from flu, but current vaccines are less effective in higher age groups, as their immune response tends to be weaker. mRNA Covid-19 jabs, however, have proven effective in older people as well as younger people.

It may be some time before mRNA jabs are available for seasonal flu. However, if H5N1 starts infecting a lot more people, and especially if we find that it is transmitting frequently between humans, there’s a chance that an mRNA bird flu vaccine could be the first such jab rolled out on a wide scale. US health officials have said that an mRNA H5N1 vaccine could be made available within weeks, if required.

Osterholm agrees that such a timeframe is realistic. The real challenge, he points out, would be getting any new H5N1 vaccine to the people who most need it. Covid-19 jabs emerged in wealthy countries and were delivered to people very quickly, he says, but “for much of the world, that wasn’t the case at all.”