I am unaware of any changes made to the definition of a vaccine on the CDC website after that date.
If the CDC made changes to the definition of a vaccine on their website in the past, the specific reasons behind it would likely depend on the context and the specific changes made. It's not uncommon for organizations to update their definitions, terminology, or guidelines as new scientific evidence becomes available, or as public health priorities shift.
Updating definitions or information on a website can also occur due to improvements in communication strategies, changes in language usage, or to ensure clarity and accuracy for the general public.
If you are referring to a specific change, I recommend consulting the CDC's website directly for the most current and accurate information regarding their definitions and guidelines on vaccines.