The USDA expanded its milk H5N1 testing program to 28 states now (65% of the country's milk). So far only California and Texas have active detections in the testing.
The USDA is also working on testing a vaccine for cows. We have a vaccine for poultry, but as the USDA notes, implementing a H5N1 vaccine for cows might be more effective and easier.
Eventually this testing program will cover the entire country. They are expanding it in steps. But so far so good.
H5N1 kills ~100% of infected chickens, but for cows the mortality rate is more like ~1-2%. Generally cows have a self limiting infection that eventually clears. It can reduce their milk output temporarily.
Biggest issue here is just that the cows can infect dairy workers and H5N1 may transmit in raw milk and raw milk products.