Catch-up immunization applies the principle that most vaccines can be administered at accelerated intervals without compromising immunogenicity. The CDC catch-up schedule defines minimum ages and minimum intervals between doses for individuals who start a vaccine series late or fall behind. Immunologic memory from prior doses is durable — the immune system retains B-cell and T-cell memory even when intervals between doses exceed the recommended schedule. Therefore, vaccine series do not need to be restarted regardless of the time elapsed since the last dose. The NP must calculate minimum intervals: for example, the minimum interval between MMR dose 1 and dose 2 is 28 days (4 weeks), the minimum interval between hepatitis B doses 1 and 2 is 4 weeks and between doses 2 and 3 is 8 weeks (with at least 16 weeks between doses 1 and 3), and the minimum age for HPV vaccination is 9 years with a 2-dose schedule if initiated before age 15 (otherwise 3 doses). For adults with unknown or uncertain vaccination history, serologic testing (e.g., hepatitis B surface antibody, varicella IgG, MMR titers) guides...