It takes just two shots of the MMR vaccine to protect a child against measles, mumps and rubella for life. The same is true for polio and hepatitis B, a few injections grant life-long immunity against these viral diseases. By showing samples of the viruses to our immune system, we teach it to store a permanent memory of these enemies and guard against them in perpetuity.
Influenza is a different matter. There is a vaccine, but we have to take it every year. That’s because flu viruses evolve at tremendous speed. They copy themselves with surprising sloppiness, producing thousands of slightly different daughter viruses. If different strains infect the same cell, they can carry out the viral version of sex by mingling their genetic material to make hybrid daughters. And occasionally, entirely new strains that we’ve never encountered before can spill over into humans from animals.
In order to prepare the immune system in advance for this constantly changing enemy, scientists have to predict the strains that are going to pose the most problems in the coming season. Dead or weakened versions of these viruses are then incorporated into a vaccine, which prepares the immune system for the year ahead. As the viruses evolve, the vaccine must be re-made and the immune system re-educated, at the cost of 2 to 4 billion US dollars every year. This strategy does save lives, but it’s not foolproof. Its effectiveness is much lower in people over the age of 65, and predictions can be wrong, leading to seasons where the vaccine underperforms. Annually, flu kills between 250,000 and 500,000 people around the world, and pandemics have the potential to kill many more. “We can do better,” says Sarah Gilbert from the University of Oxford, UK. “The vaccines we use for flu are really using decades-old technology. There’s nothing else we vaccinate against every year.”
You could argue that we’d be in better straits if it wasn’t for the emergence of another virus that is adept at outmanoeuvring the immune system - HIV. Once HIV was discovered in the early 1980s, “the immunologists who’d done really interesting work on flu in the 1970s started working on that instead,” says Gilbert. “Nothing really very innovative was done with flu for a while.” The presence of the annual vaccine eased some of the pressure and, after all, there hadn’t been a flu pandemic since 1978.
But the last two decades have shaken us out from under this blanket of false security. In 1997, an outbreak in China heralded the spread of H5N1 bird flu into humans. To our knowledge, the virus has so far infected 622 people and killed 60% of them. In 2009, a strain of H1N1 leapt from pigs into humans, triggering the first pandemic for 30 years. And just this year, H7N9 has emerged out of nowhere to infect more than 130 people in China. “We can’t really predict what’s going to come,” says Ian Wilson from the Scripps Research Institute in La Jolla, USA. “But if we had a universal flu vaccine, we could counter all of [those strains] and not worry about a pandemic.”
When flu viruses enter our bodies, they recognise and enter cells using a protein that studs the surface like pins in a cushion called haemagglutinin (HA). When our immune cells see these intruders, they start producing antibodies that recognise HA and stick to it, disabling it and preventing infections. This is why flu vaccines use real viruses - so the immune system can make the right antibodies ahead of time. The problem is that there are 17 different types of HA (an H5N1 virus, for example, carries the 5th subtype) and many different strains within each type. An antibody that neutralises one may not work against another.