It's been in cows since last March. That's why all the ruckus about "muh rights to drink raw milk!" They were told not to (by the libs) because of bird flu, so... here we are.
This is happening specifically in US because of its intensive dairy/poultry practices. It is a CDC matter. There happens to exist a chicken vaccine, but "concerns over exports of broiler chickens complying with foreign rules" stops its use. No idea if it makes the chickens unsafe to eat or if it's just never been tested.