Immunological Memory in Viral Infections: Lessons from COVID-19

Abstract

Immunological memory is basically the immune system’s way of not starting from scratch every time it sees a virus. Once the body has gone through that first encounter, the next one is usually quicker and more effective-though how well this works can depend a lot on the virus itself. B cells and T cells are the central players here, but over the past few years scientists have noticed that even some innate immune cells can be “trained” to respond a little better the second time. The strength and the durability of this memory, however, are uneven. Some viruses, like measles, give protection that pretty much lasts a lifetime. Others, such as the seasonal coronaviruses, don’t leave much of a lasting impression at all, which is why people can keep catching them. COVID-19 came along and forced researchers to study these differences in real time. Both infection and vaccination against SARS-CoV-2 create a layered form of immune memory-antibodies at first, but also memory B cells and T cells that stick around. The antibody levels fade within a few months, but the cellular memory seems to last longer and has been especially important in preventing serious disease. Vaccination, especially with the mRNA platforms, has turned out to be very effective in building this long-term memory. And when people have both infection and vaccination- so-called hybrid immunity- the protection is broader and more durable than either alone. That said, the story isn’t finished. People respond differently depending on age and health, the virus itself keeps mutating, and the current vaccines don’t really boost mucosal immunity in the respiratory tract. Understanding all of this in the context of COVID-19 doesn’t just help with today’s vaccine strategies- it also gives clues for how we might handle whatever virus shows up next.