Introduction

Coronavirus disease 2019 (COVID-19)-a respiratory illness caused by a recently identified, highly transmissible virus known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV- 2)-has had considerable global morbidity and mortality [1,2]. As a result, a number of anti-SARS- CoV-2 vaccines have been developed with unprecedented speed [3]. While these vaccines have satisfactory efficacy and safety profiles, experience with their use is undeniably limited [4,5]. Accordingly, additional knowledge of rare side-effects is important to inform future recommendations.

Hepatitis B is a highly contagious liver infection caused by the hepatitis B virus (HBV), a virus transmissible via contact with contaminated bodily fluids, including blood, saliva, semen, or vaginal secretions; in addition, it can also be transmitted to an infant during childbirth [6]. Up to 30% of the global population is believed to have a current or previous HBV infection [7]. Hepatitis B surface antigen (HBsAg) is the most common marker of acute HBV infection and is detectable between 1-9 weeks of exposure to the virus, with decreasing HBsAg concentrations indicative of resolving viremia.8 Individuals who have achieved immune control of HBV infection will typically show HBsAg negativity approximately 15 weeks after infection onset [8].

In 1981, the first HBV vaccine was approved for human use [7]. Transient antigenemia is known to occur for up to two weeks after HBV vaccination, thereby leading to a false-positive diagnosis of HBV infection [9,10]. However, there are as yet no reports of hepatitis B antigenemia following SARS-CoV-2 vaccination. Herein, we report two cases of isolated HBsAg positivity after COVID- 19 vaccination in the absence of other HBV markers, symptoms, risk factors, or recent vaccination.