How to know if you have a reaction to a shot if you've never had the shot before?

You might...: ...Have had a reaction to something similar. Very severe egg allergy can sometimes cause a reaction to egg-containing vaccines, but it would have to have been a life-threatening reaction to a tiny quantity of egg, not a rash after an omelet.

You can't know: Every time you have an injection there is a possibility of reaction. This likelihood is extremely low especially if you have not been exposed to any of the components. However, it is very unlikely that anyone in the us has not been exposed to components through prior vaccination / injection. Despite this it is very rare to have serious reaction. Most reactions are mild and local.