Why we're asked about eggs when we get the flu shot

When we receive the flu shot (influenza vaccination) we're typically asked a few things, such as are we currently sick, have we ever had Gullian Barre' Syndrome (GBS), and are we allergic to eggs. We're asked if we're sick because the vaccine triggers our immune system, and if we're sick, our immune system is already busy. Depending on how sick we are, perhaps it's better to allow our immune system to finish off our illness before presenting it with the vaccine. We're asked about GBS because having it once suggests that we may get it again. I discuss GBS and the flu vaccine in more detail here. Eggs are my focus on this post, and that explanation requires a little bit of background on how a virus (such as the flu) reproduces (copies itself).

A Little Virology (Study of Viruses)
A cell, whether it's one of the cells in our bodies, or a cell from our pet cat or dog, or whether it is a bacterium (1 bacterium + 1 bacterium = 2 bacteria, and each bacterium is a single cell) contains everything it needs to reporduce. All we need to do is feed the cell, and it does the rest - it repproduces its DNA and all of its internal parts, and then splits into two new cells.

A virus can't do this on its own. It lacks some of the machinery needed to reproduce, and needs to break into a cell and trick the cell's internal machinery into reproducing the virus.

Making the Vaccine - the Chicken Egg Connection
The flu vaccine works by showing a weak or inactivated copy of the flu virus to our immune system, so that if we later see the real flu virus, the immune system can quickly pounce on it and kill it off. As I've said before (see this post for details), the vaccine is kind of like a test prep course for our immune system, where the flu is the actual test: taking the vaccine leaves our immune system better able to take the flu.

The point for this post is that in order to make the vaccine, we need to make a lot of copies of the flu virus, and the easiest way for us to do that is to put the virus in a cell and let the virus do its thing, hijacking the cell and making lots of copies of itself. The cell we use for this is a chicken egg (a chicken egg is a cell).

Since we grow the virus up in a chicken egg, we can end up with some egg in the vaccine itself. So, if someone is allergic to egg, then perhaps the vaccine isn't for them.