Here is a little more info on this subject of blackleg from the Alabama Cooperative Extension
What is blackleg?
Blackleg has been recognized as a livestock disease since before medieval times, and today we often use the term loosely to describe several diseases caused by organisms in the Clostridium class of bacteria. However, there are more than 60 different types of Clostridium bacteria, and not all cause disease.
What we commonly call blackleg is a highly fatal infection caused by Clostridium chauvoei, resulting in a gas gangrene in the muscle of young cattle, usually occurring between 4 months and 2 years of age.
Blackleg seldom affects cattle older than 2 years of age, most likely due to immunity induced by vaccines or natural exposure.
However, sporadic cases do occur in cattle older than 2 years and are often associated with the reuse of needles for multiple injections.
Blackleg can also be a problem in cattle less than 4 months old that do not receive adequate passive immunity through colostrum.
"adequate passive immunity" can also come from medicated milk replacer and medicated calf starter feed.
From that you can draw your own conclusion as to the necessity to vaccinate/worm or not under 4 months.
I vaccinate/worm at weaning ( between 4-7 months ) for calves for replacement or to be sold as certified precondition. I don't vaccinate/worm calves to be sold not certified preconditioned.
liz