Background: Hepatitis A is endemic in many countries. Swiss guidelines recommend vaccinating patients native from endemic areas. In Geneva's Children's hospital, migrant children are screened and vaccinated if seronegative. Because hepatitis A's prevalence is decreasing worldwide, more children are seronegative at arrival, highlighting the need for immunization in medical centers and refugee camps and questioning the benefits of systematic serology. Other Swiss hospitals vaccinate regardless of serostatus. This study's aim is to assess migrant children's immunity according to origin and age, and the cost-effectiveness of different immunization strategies. Methods: We retrospectively analyzed 329 children's serostatus (1-16 years of age) between 2012 and 2015, using enzyme-linked fluorescent assay method. Serology and vaccine costs were based on local prices. Groups were compared with χ test and the age-seropositivity relationship was studied with linear regression. Results: The predominant regions were the Eastern Mediterranean and European Regions with mostly negative serologies (71% and 83%) and the African Region with mostly positive serologies (79%). Immunity varied depending on birth country. Regardless of region, seropositivity increased with age (P < 0.001). The most cost-effective vaccination strategy was an individualized approach based on age and origin, reducing costs by 2% compared with serology-guided immunization and by 17% compared with systematic vaccination. Conclusions: Many migrant children >5 years old are seronegative and at risk of clinical infection. They need to be immunized. New guidelines according to age and origin should be defined to reduce immunization costs. We recommend systematic vaccination for patients <5 years old or native from low endemicity areas (≤25.7% of seropositivity). For the others, we propose serology-based vaccination.