Background: Long term immunity of Hepatitis B vaccine, the first vaccine to prevent cancer is a debate. Objective: to assess anti-HBs level in previously vaccinated Health Care Workers (HCWs) and Medical Students (MS). Also, determine the effect of a booster dose on humoral and cell mediated immunity in non seroprotected subjects. Methodology: A cross sectional study included 120 vaccinated HCWs and MS. HBsAg, Anti-HBs level and Interferon γ (IFN-γ) were assessed. Anti-HBs and Interferon γ (IFN-γ) levels were reassessed after a booster dose to non seroprotected subjects. Results: 90% of HCWs and 23.3% of MS were seroprotected. Boosted HCWs and MS showed significant increase in Geometric Mean Titer (GMT) of anti-HBs level (P=0.028, P<0.001 respectively), while IFN-γ mean concentration level (MCL) increased significantly in MS group only (P=0.01). Conclusion: AntiHBs level decreases significantly over time. Long term memory was found to be still intact after a booster dose.