Someone asked me if the COVID19 vaccine could cause them to have a positive COVID test, which wasn't something I had considered before. Short answer: no. Full answer: 1/7

Here is what the COVID virus' genome looks like: it is very small and has only a few genes (coloured blocks) (for comparison, we have about 20,000 genes). The COVID tests target the purple N gene at the end. 2/7

Humans don't have an N gene in their cells normally, so if the test picks up the N gene, it's COVID.
The COVID vaccines are a synthetic construct that carries the RNA for the S gene (yellow block) 3/7

(the S gene codes for the spike protein that makes the COVID particle look like a spiky ball, and gives it the name "coronavirus" since the spikes look like a little crown under electron microscopes). 4/7

The S gene was chosen for the vaccines because the protein is on the outside of the viral particle - so our immune system could recognize it easily. The vaccines in use today carry the instructions for this one tiny part of the virus. 5/7

If the primers for the COVID tests targeted the S gene too, or if the vaccine carried instructions for the N gene, then there might be cross-over, where a test picked up the vaccine-introduced gene and gave a false positive. 6/7

But scientists had thought this through and the tests do not target the same area, and as no portion of the N gene is in the vaccine, there should be no cross-reactivity between the vaccines and the COVID tests. [fin]