system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do antibody tests differentiate between antibodies generated by a COVID-19 infection versus those generated by the vaccine?

I recently participated in a study for which I took a COVID-19 antibody test to see whether I had ever contracted the virus. As I've been vaccinated, I assumed this would confound the test, but in my informational material, it says, "The tests that we use do not identify antibodies from vaccine (only from past infection), so if you have had the vaccine, do not be concerned that this antibody test result is negative."

How does such a test work & how does it differentiate between these two types of antibodies?
gpt
The phrase you're looking has a name: "DIVA" - distinguishing infected from vaccinated.

E: more ELI5 here, for an ELI15, see further below. 

Imagine that anyone who gets the vaccine gets a red Lego. Anyone who gets infected gets a red Lego *and* a blue Lego. 

If your test looks for the red Lego, it can't tell people apart, because both infected and vaccinated people got a red Lego.

If you have a test for the blue Lego, it can tell who was infected because only infected people got a blue Lego. 

**ELI15:**

For covid, the main vaccines are against the spike proteins, and your body makes antibodies against the spike protein.

There are a number of antibody tests for covid. Some test for spike protein antibodies - these cannot tell you if a person was infected or vaccinated. They are not Diva-compatible. 

Some tests look for antibodies against another viral protein - the nucleoprotein. These tests are Diva-compatible and can tell the difference between someone who's been vaccinated and someone who's been infected, because only the infected people will have antibodies against the nucleoprotein.