Unlike fine wine, the human body does not improve with age. Hearing fades, skin sags, joints give out. Even the body’s immune system loses some of its vigour.
This phenomenon, known as immunosenescence, might explain why older age groups are so hard-hit by COVID-19. And there is another troubling implication: vaccines, which incite the immune system to fight off invaders, often perform poorly in older people. The best strategy for quelling the pandemic might fail in exactly the group that needs it most.
Scientists have known for decades that ageing immune systems can leave the body prone to infection and weaken their response to vaccines. In June, the US Food and Drug Administration announced that a COVID-19 vaccine would have to protect at least half the vaccinated individuals to be considered effective, but protection in older adults might not even meet that bar. “No vaccine is going to be as effective in the elderly as it is in young people,” says Matt Kaeberlein, a gerontologist at the University of Washington in Seattle. “That’s an almost certainty.”
The human immune system is mind-bendingly complex, and ageing affects nearly every component. Some types of immune cell become depleted: for example, older adults have fewer naive T cells that respond to new invaders, and fewer B cells, which produce antibodies that latch on to invading pathogens and target them for destruction. Older people also tend to experience chronic, low-grade inflammation, a phenomenon known as inflammageing (see ‘Depleted defences’). Although some inflammation is a key part of a healthy immune response, this constant buzz of internal activation makes the immune system less responsive to external insults. “This overarching, chronic inflammatory state is what’s driving much of the immune dysfunction that we see,” says Kaeberlein. The upshot is a poorer reaction to infections and a dulled response to vaccines, which work by priming the immune system to fight off a pathogen without actually causing disease.
