@sun @jeffcliff
That's a possibility, now that you mention it.
I hadn't considered ineffective vaccines as a vector of transmission, I would have hoped dairy farmers would be careful of what they're giving their cattle.
Come to think of it, raw milk in my state is illegal except under very specific circumstances so I'm confident that the few instances where raw milk is accessible would have better cared for cattle.
I'll have to dig into their dataset when I have time.