Whether we're talking about your toddler or you are tracking your own healthcare (yep - adults get immunizations, too!), knowing where to find those records is important. Schools and sport teams need access to this information. Adults have their own lists of immunizations, too, for the shingles vaccine and more, and you never know when you're going to need access to this information on short notice.
When my kids were younger we got little booklets to record the type and date of immunizations, as well as the name of the Dr. who gave them. I kept them in the file cabinet under each of their names, along with other medical information.
In the digital age, we have a plethora of options! Parents can use the little booklet the Drs are still handing out, or use an online printable like the ones provided by the Center for Disease Control and Prevention (CDC) - they can be printed out or saved as a PDF and filed as such. And, of course, there are many apps for tracking immunizations, such as Shots (love the logo with the needle - NOT!). The Shots app has the complete set of vaccine schedules from the CDC, along with up to date information about risks, adverse reactions, contraindications, etc.
No matter how you choose to record immunizations, remember WHERE you keep them, and keep them up to date. Healthcare record keeping shouldn't be a shot in the dark!