Maybe worthless was not the right thing...but a false sense of security?? If the mother's immunity overides the effectivness of the vaccine...then you have an 8 week old puppy that was given a vaccine at 6.5 weeks of age...and think it's got some immunity. The dam's immunity is wearing off at this age...and the vaccine immunity was deleted...so you really have a puppy at 8 weeks of age with no immunity....or very little.
The problem is that some pups hold on to their mother's immunity longer, and some shorter....and you can't tell unless you do a titer. Titers around here are $100. a draw....so most vets push vaccinating.
If the vaccine is not given in the proper intervals...then they do not create immunity properly. This is why some puppies develop parvo even though they have been vaccinated.
Gina