How Vaccines Work

Medical students who sit in with me hear the same things over and over. It’s true that I have a few set lines and spiels that I find trusty and reliable and flow off my tongue several times a day. This goes for both questions I ask and explanations I give. One of the aims of this blog is to write down some of those explanations and discuss some of those lines (and maybe improve them with a bit of feedback). The first that I’m putting down is my explanation of how vaccines work.

There are 2 types of vaccines: ‘live,’ and ‘inactivated.’ Now, imagine a germ is a soldier going off to fight the body. The idea of a vaccine is to alert the body’s immune system to that soldier, so that, if seen again, it can be recognised easily and killed off before it does any damage. (Immune systems have pretty good memories and are very good at killing off stuff they have seen before.)

So called ‘live’ (more accurately, ‘live attenuated’) vaccines work by getting one of the soldier germs and beating it up thoroughly to take the fight out of it. This is then shown to the immune system (normally by injecting it), and the immune system fights it off with ease, attaining memory of the germ, and hence immunity, in the process.

The live vaccines tend to give bigger immune responses which last longer, but can, as the vaccine was a live, if beaten up, soldier, cause more of an associated illness and fever whilst that immune response is developed (basically a mild version of the illness being immunised against).

‘Inactivated’ vaccines, on the other hand, work by just tearing off a badge from the soldier’s uniform, showing that to the immune system, and telling it to bash whatever is wearing that badge if it ever sees it again. These vaccines tend to need more boosters to give long-lasting immunity, as they are simpler and the immune system doesn’t mount a massive reaction to the badge, because a badge alone doesn’t pose much of a threat. However, they also tend to give smaller adverse reactions, with the side effects normally confined to soreness around the injection site, some irritability in kids given them, and a mild fever.

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