Recent (last 5 years or so) investigative work done, looking at immune response/capabilities in neonatal calves, indicates that their immune systems kind of 'bottom out' between days 3 and 7 - so...if you're going to give a vaccine, do it in the first day or so or wait 'til they're 8-10 days old.
Giving 'em during that 'immunologic downturn' period between days 3-7 probably won't hurt anything - but the likelihood that they'll be able to respond appropriately is iffy, at best.
It's a good product; have been using it in our pre-weaning vaccination protocol for the past 2-3 years.
No respiratory issues in baby calves here, so we don't vaccinate then. But, they CAN respond if you give it when their immune system is ABLE TO respond.
Also, it looks like 'maternal antibodies' obtained in colostrum, don't really interfere with calves' ability to respond to most of the commonly-used vaccines.