False positive hepatitis B surface antigen after recent vaccination in pregnancy: a diagnostic pitfall

We report the case of a 38-year-old pregnant woman who presented with a reactive hepatitis B surface antigen (HBsAg) during routine prenatal screening. Previous tests had been negative, and the patient expressed concern about possible recent infection and vertical transmission. Further evaluation revealed a recent hepatitis B vaccination four days before the test. Repeat serology performed two weeks later showed negative HBsAg, negative anti-HBc, and high titers of anti-HBs, consistent with vaccine-induced immunity. Transient antigenemia following immunization is a recognized phenomenon that may lead to misinterpretation, unnecessary referrals, increased costs, and anxiety, particularly in pregnancy. This case highlights the importance of considering recent vaccination history and performing complete serologic panels to establish an accurate diagnosis. Practical implications for clinicians involved in prenatal care are discussed.