In a hypothetical alternative universe where we already have a vaccine against Covid-19, world leaders will have a choice about how to deliver it to the population. The most vulnerable people, along with the nurses, doctors and care workers who look after them, are likely to be protected first.
If only it were that straightforward. The most vulnerable age group, the elderly, are particularly tricky to vaccinate.
“We have very few vaccines designed for older populations,” says Shayan Sharif, a professor of vaccinology at the University of Guelph, Canada. “More often than not in the last century, most vaccines have targeted childhood diseases.”
Shingles is one exception, usually given to patients in their 70s, and there are one or two other vaccines for diseases like meningitis or human papillomavirus developed for young adults. But otherwise immunology is skewed in favour of children.
“We have a tremendous amount of knowledge about childhood diseases,” says Sharif. “When it comes to young adults, middle age and old age, we don’t have a lot of experience.”
To understand why older people are harder to vaccinate, we have to look at the differences in their immune system. Many infectious diseases are more severe in older adults than younger adults. Older people have more risk factors – a lifetime of exposure to carcinogens or other infectious diseases will increase the risk of future disease from new infections. But they also undergo something called immunosenescence – ageing of the immune system.
Just like many other parts of the body, our immune system shows signs of our ageing. Some of the immune cells lose their function. The immune system is a very complex network of cell types that interact with each other. If something, somewhere within the system is not working, it interrupts the delicate balance of the immune response.
How does the aging immune system work?
When you are infected by a pathogen, the first layer of the immune system, the innate immune response, starts attacking the pathogen at the site of infection. For respiratory diseases, that could be the lungs, trachea or nose. White blood cells, or macrophages, attack the pathogen, swallowing it up before destroying it.
As those macrophages break apart the pathogen inside themselves, they present bits and pieces of it to another type of immune cell known as T cells. These serve as the “memory” of the immune system. T cells cannot see the pathogen by themselves and need certain macrophages, called antigen presenting cells, to show them the pathogen. That activates the next layer, the adaptive immune system.