Irigoyen, MatildeLaRussa, PhilipFindley, Sally EChen, ShaofuCaesar, ArturoTesler, PeterBackground: In 2002, the Advisory Committee on Immunization Practices (ACIP) recommended vaccine doses administered </=4 days before the minimum age or interval be counted as valid. The study objective was to assess the impact of the 4-day grace period on the need for revaccination and associated costs in a low-income community, compared to standard practice (i.e., repeating all doses that fall outside current ACIP guidelines). Methods: From 1999 to 2001, semi-annual immunization assessments of 8293 randomly selected children, aged 19–35 months, were conducted at a 16-practice network serving an underserved community in New York City. Outcome measures were rates of antigen-specific invalid doses and number of children needing revaccination, with and without the 4-day grace period. Revaccination costs were based on the Vaccines for Children (VFC) price list. Results: The 4-day grace period reduced the number of children needing revaccination from 17.1% to 12.0%, a drop of 30%. The rates of invalid doses decreased from 1.9% to 1.3%, a drop of 33%. Invalid doses for hepatitis B (HepB)-2 decreased by two thirds (69.7%); for diphtheria–tetanus–acellular pertussis (DTaP)-1, Haemophilus influenzae type b (Hib)-1 and Hib-3 by half (44.9%–50.0%); for Polio-1, Polio-2, Polio-3, and measles–mumps–rubella (MMR) by one third (31.6%–33.3%); and for DTaP-2, DTaP-3, HepB-3, and varicella by nearly one quarter (20.0%–24.0%). At these rates, revaccinating 100,000 children younger than age 3 years would cost $213,588 per year, compared to $152,539 with the 4-day grace period, in vaccine costs alone. Conclusions: In a low-income community, ACIP’s 4-day grace period made a significant impact on the number of children requiring revaccination and on revaccination costs. However, the number of children needing revaccination remains high.PharmacotherapyPracticeinterventionsResearchadvisory committeeimmunizationlow-income communityAdvisory Committee on Immunization PracticesACIPArticle