How does a vaccine work to protect us?

This week COVID-19 vaccines are going under our microscope; we’ll hear how the different vaccines work, how they have been tested and what 90% or 70% efficacy actually means, how scientists know whether they work against viral variants or not, and whether vaccination passports are on the cards in future...But first, how do vaccines actually work? Katie Haylor explains...

Katie - Vaccines work by showing the immune system what a foreign invader looks like before we meet it for real. This enables us to build an immune response, made up of white blood cells, called T cells, that can recognise and kill-off cells harbouring viruses, and sticky molecules called antibodies that can glue themselves to critical parts of infectious organisms, preventing them from causing further trouble. To make this happen, the body needs to be exposed to either the whole virus or bacterium, or to some of the critical pieces that make that infection tick.

It takes about 2-3 weeks after vaccination for the response to begin to develop. The immune system also stores the instructions for how to make this response in a population of “memory cells”. These are very long lived, so if we encounter the same threat again at some later time we can often use this memory to rapidly mobilise the same effective response. Crucially though, this time, it happens much faster, so we get on top of the infection before it gets on top of us. This is the “immunity” that we get from a vaccine. But like anything that activates the immune system, vaccines can sometimes produce side effects similar to the infection itself. Chills, headaches and muscle aches sometimes happen, but these are almost always mild and short-lived.