We do give pneumabort-K to bred mares, and have not yet had any horses get a reaction to the vaccine. No lumps, no stiff necks, nothing.
For those that say they have quit giving the shots and haven't noticed any difference in the number of abortions....of course you won't see any difference, up until the year you actually get a rhino infection in your herd and you have an abortion storm and lose 50-70% of your foal crop. if your mares are never exposed to rhino then the pneumabort-K vaccine will not "help"--if you are vaccinating your mares and still having abortions, it is almost certain that those abortions are due to something other than rhino.
if you're having early abortions, the cause is probably not rhino; rhino causes late term abortions. keep in mind that even if the mare is infected with the virus early in the pregnancy, she does not abort until late in the pregnancy--that is the nature of the virus. She may even carry full term, but if the foal was infected with rhino he will be born dead or dying--he will have internal lesions that make it impossible for him to survive.
If you only raise one or two foals a year it may be difficult to tell that you've had an abortion storm--if you were raising 20 it would be much more obvious that your losses are due to rhino if one year you lose 50-70% of those foals. I do know people who have had rhino go through their herd, with the result being massive losses. A QH breeder just down the road lost 60 or 70 of 100 foals one year--it was a huge loss for him; that was a lot of years ago and he's scaled down his operation a lot in the last few years, but he has given the pneumabort-K vaccines every year since then. Very expensive for sure, but cheap insurance against a repeat of that disasterous season.