Chronic hepatitis B infection remains a global health threat despite vaccination program.\[1\] Nearly 3 billion population are infected with chronic hepatitis B infection globally. In China, the pooled prevalence of chronic hepatitis B infection was around 7% in year 2018, making an estimated 84 millions of population living with chronic hepatitis B infection. Hepatitis B virus infection is the leading cause of liver cirrhosis and hepatocellular carcinoma, leading to significant morbidity and mortality. Patients with hepatitis B infection can present with a wide range of clinical presentation, ranging from inactive carrier to fulminant hepatitis, cirrhosis or development of hepatocellular carcinoma. Besides, a significant proportion of patients are suffering from occult hepatitis B infection (OBI), which is defined as a state of undetectable serum HBsAg yet detectable serum and/ or intrahepatic HBV DNA. Report from the European Association for the Study of the Liver (EASL) in 2008 has revealed that around one-third of the global population has serological evidence of past or present HBV infection. Disappearance of HBsAg, also known as HBsAg seroclearance, can occur either spontaneously or after anti-viral treatment. However, patients with past or resolved HBV infection are still at risk of development of hepatocellular carcinoma, liver cirrhosis and liver-related mortality.The reported prevalence of OBI amongst patients with non-alcoholic fatty liver disease in Hong Kong was 35% (submitted to American Journal of Gastroenterology, under review). It is well known that there is risk of HBV reactivation after chemotherapy or immunosuppressive therapy which can lead to fulminant liver failure or even death. Short course of high dose corticosteroid could increase the risk of hepatitis flare in patients with chronic hepatitis B infection. Even in patients with OBI, a high daily dose of corticosteroid (\>40mg prednisolone equivalents) increased risks of hepatitis flare. It is thus important to know the hepatitis B virus status before start of chemotherapy, immunosuppressive therapy or corticosteroid.

Biologics is one of the mainstay treatments for many autoimmune diseases, including inflammatory bowel disease (IBD), rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE), psoriasis etc. The incidence of IBD has increased rapidly in Hong Kong over the past 30 years. The age-adjusted incidence of IBD in Hong Kong has rose from 0.10 per 100,000 individuals in 1985 to 3.12 per 100,000 individuals in 2014. The reported prevalence of RA in Hong Kong was 0.35% while the reported prevalence of SLE in Hong Kong was 0.1% with annual incidence of 6.7 per 100,000 individuals. It is well known that treatment with biological agent like anti-CD20 monoclonal antibody (rituximab) in patients with OBI can lead to fulminant hepatitis B flare. Recently, anti-tumor necrosis factor alpha (TNF α) has been widely used in patients with inflammatory bowel disease and various rheumatological diseases. Risk of hepatitis B reactivation after anti-TNF α has been reported, but actual risk remains controversial. In a report from Japan including 135 rheumatoid arthritis patients with previous HBV infections, 17/135 (12.6%) had HBV reactivation after receiving immunosuppressants and patients who were treated with biologics were more frequent to develop HBV reactivation compared to those who were not on biologics. In another study, 9/168 (5%) anti-HBc positive patients developed HBV reactivation after receiving anti-TNF α and one patient died due to fulminant liver failure.However, another three studies revealed low rates of HBV reactivation (0-2%) amongst rheumatology patients with previous HBV infections receiving anti-TNF α. The European Crohn's and Colitis Organisation (ECCO) suggested routine antiviral prophylaxis for patients with previous HBV infection starting biologics is not recommended. Instead, regular monitoring of ALT/ AST, change in HBV serology and HBV DNA every 1 to 3 months should be performed. In 2015, the American Gastroenterology Association (AGA) has developed guideline on the prevention and treatment of hepatitis B virus reactivation during immunosuppressive therapy. It classifies patients receiving anti-TNF α as moderate risk of developing HBV reactivation and antiviral prophylaxis is preferred over monitoring alone, but it is a weak recommendation. Therefore, it is now still uncertain whether prophylactic anti-viral should be given for patients with previous HBV exposure before starting anti-TNF α.

With the advancement of technology, there are now more biological agents and small molecules for the treatment of IBD and various rheumatological conditions. These include anti-integrin, cytokine inhibitors, and small molecules e.g. JAK inhibitors. Cytokine inhibitor, ustekinumab (anti-IL12/23), has been reported to be leading to hepatitis reactivation in a psoriasis patient who has past HBV infection.\[26\] Whether these agents can lead to hepatitis flare in patients with past HBV exposure remains largely unknown.

Based on the compelling data, the investigators hypothesize that use of biologic modifier therapies in patients with occult hepatitis B infection will lead to hepatitis flare. The investigators aim to assess the rate of hepatitis flare while on biological modifier therapies in both retrospective population-based database and prospectively in patients who are on biological modifier therapies.