Once someone is on active duty, the military healthcare system (TRICARE) monitors required vaccinations same as any healthcare organization. On I-Day, one of the stations will check immunizations and document status and needs.
This brings to mind one of the age-old bits of wisdom about checking into and out of new duty stations, which involves executing a checklist of running around, including visiting a healthcare facility. Check on your vax status a few weeks before heading out to new duty station! There is nothing a junior corpsman (enlisted medical personnel) likes better than to say, “Sir, ma’am, I can’t sign you out until you have your yellow fever and TDAP.” Nothing like starting a XC drive to next duty station feeling whacked by sore arms and an achey body.
@Devil Doc I am not imagining the faint bwah ha ha I thought I heard when this happened?