This is a difficult topic to research these days due to the anti-vax/anti-anti-vax noise. :(

It doesn’t appear that horizontal transfer is absurd, and while I’m not immediately seeing a large body of evidence that it happens all the time, it does seem to be generally accepted that it occurs.

Here’s what I believe to be an informative article on the topic: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3667938/