The epidemiological picture painted by the review is strikingly heterogeneous. In the general population of low-endemicity countries, OBI prevalence can be as low as 0.8 percent. But in groups with concurrent viral infections the figure climbs steeply: more than 10 percent of individuals co-infected with hepatitis C virus or HIV carry occult HBV. The rate exceeds 40 percent among people with an isolated positive antibody to hepatitis B core antigen, a serological fingerprint of past exposure that persists long after surface antigen vanishes. A systematic review cited in the article found occult infection in adults across every region studied, and separate analyses in children show that babies born to surface-antigen-positive mothers can acquire OBI despite vaccination and hepatitis B immunoglobulin prophylaxis. These numbers matter because they define the size of the reservoir from which transfusion transmission, reactivation and, potentially, liver cancer can emerge.

Understanding how OBI arises requires a brief excursion into HBV biology. The virus maintains a persistent intermediate known as covalently closed circular DNA, or cccDNA, a minichromosome that resides in the nucleus of infected hepatocytes and serves as the transcriptional template for all viral RNAs. When surface antigen seroclearance occurs, whether spontaneously, after antiviral therapy or following recovery from acute infection, cccDNA is not eradicated. Instead it is transcriptionally silenced by epigenetic mechanisms, including histone deacetylation and methylation changes orchestrated by host factors such as SIRT3 and opposed by viral HBx protein recruits like LSD1 and Set1A. Integrated HBV DNA in the host genome can also persist independently of cccDNA, and both forms have been detected in the livers of patients classified as serologically recovered. The review emphasises that OBI therefore represents a spectrum of viral persistence rather than a single entity, arising either after HBsAg seroclearance in formerly chronically infected patients or after resolution of acute infection.

Why does surface antigen fall below detectable levels while viral DNA persists? The review details several converging mechanisms. Mutations in the PreS/S gene of the virus can alter or truncate the surface protein so that commercial assays fail to recognise it, a phenomenon known to account for a subset of occult infections. Viral variants carrying deletions in the X gene have been found in vaccinated individuals with OBI, suggesting immune selection pressures shape the hidden viral population. Host epigenetic silencing of cccDNA suppresses surface antigen expression to vanishingly small amounts, and newer ultrasensitive assays have shown that some patients previously labelled seronegative in fact have extremely low-level surface antigen detectable at concentrations below the standard cut-off of 0.05 IU/ml. This observation carries a conceptual twist: with sufficiently sensitive assays, some occult infections might be reclassified as overt infection, blurring the boundary between the two states.

Diagnosis remains the most technically demanding aspect of OBI. Serum HBV DNA levels in occult infection are typically extremely low and fluctuate over time, often dipping below the detection limits of routine commercial polymerase chain reaction assays. Hepatitis B core antibody, the single most useful serological marker, indicates prior exposure with a sensitivity of 77 percent and a specificity of 76 percent for seropositive occult infection, but it cannot by itself confirm active viral persistence. Liver tissue offers the definitive answer, since intrahepatic cccDNA is the hallmark of the condition, yet liver biopsy is rarely justifiable for diagnosis alone. The methodological frontier lies in droplet digital PCR, which partitions samples into thousands of nanolitre-scale reactions and enables absolute quantification of scarce DNA templates. Recent studies have validated high-sensitivity droplet digital PCR assays for both serum HBV DNA and intrahepatic cccDNA, and the review argues these platforms are reshaping what clinicians can detect. Complementary biomarkers such as hepatitis B core-related antigen and serum HBV RNA, which reflect cccDNA transcriptional activity, are being evaluated as non-invasive windows into occult viral replication.

The clinical consequences of undetected occult infection fall into three major categories. The first is transmission. Documented cases show HBV has been transmitted through transfusion of blood products and through solid organ transplantation from donors with occult infection, sometimes with devastating outcomes for immunosuppressed recipients. Analysis of transmission events has forced a revision of the minimal infectious dose, revealing that even very low viral loads in donated blood can establish infection. Nucleic acid testing of blood donations has reduced but not eliminated this risk, and the review discusses the ongoing international debate over whether anti-HBc screening of donors should be adopted more widely, weighing improved safety against the loss of otherwise suitable donors in endemic regions.

The second category is reactivation. Patients with occult HBV who receive immunosuppressive therapy, particularly B-cell-depleting agents such as rituximab, but also corticosteroids, ibrutinib, abatacept, temozolomide and therapies used in haematopoietic stem cell transplantation, are at risk of reverse seroconversion, in which surface antigen reappears and viral replication surges, occasionally causing fulminant hepatic failure. Quantification of core antibody titres may help stratify risk among patients with resolved infection, and international guidelines from the AASLD, AGA and EASL converge on the recommendation that individuals with serological evidence of prior HBV exposure should receive prophylactic nucleoside analogue therapy before high-risk immunosuppression. The review stresses that occult carriers are precisely the patients in whom routine serological screening fails, underscoring the importance of anti-HBc testing before oncological and rheumatological treatments.

The third and most debated consequence is oncogenesis. In patients with cryptogenic cirrhosis and cryptogenic hepatocellular carcinoma, studies have repeatedly found occult HBV DNA, cccDNA and viral integration events within tumour and surrounding liver tissue. A high proportion of patients with undetectable surface antigen and hepatocellular carcinoma harbour HBV DNA integrated into hepatocyte genomes, sometimes without cirrhosis, and integrations that reshape genomic structure have been shown to promote carcinogenesis through insertional mutagenesis and dysregulation of oncogenes such as those on chromosome arms recurrently targeted in the cancer genome surveys. The review also describes associations between occult infection and accelerated fibrosis in chronic hepatitis C, more severe outcomes in non-alcoholic fatty liver disease, and a prediction of non-alcoholic steatohepatitis in severely obese individuals. Nevertheless, the authors are careful to note that routine OBI screening is not recommended for milder chronic liver disease of other causes, and that the oncogenic potential of occult infection requires further study before screening policies change.

Management of confirmed OBI follows a deliberately targeted approach. The review recommends that OBI be excluded in individuals presenting with cryptogenic cirrhosis or hepatocellular carcinoma, and that treatment with a nucleoside analogue be initiated if occult infection is confirmed. For transplant medicine, grafts from anti-HBc-positive donors can be used with appropriate antiviral prophylaxis and monitoring, a strategy supported by systematic reviews showing acceptable recipient outcomes in liver, kidney and heart transplantation. For blood services, nucleic acid testing in minipool or individual-donation formats, alongside cost-effectiveness modelling from centres such as Shandong Blood Center in China, informs the evolving balance between transfusion safety and blood supply. What remains conspicuously unresolved is the management of the millions of anti-HBc-positive individuals worldwide who have no evidence of active replication and no need for treatment, yet who carry a dormant viral archive whose clinical behaviour over a lifetime is only beginning to be characterised.

The review closes with a set of challenges that will define the next decade of occult HBV research. Ultrasensitive surface antigen assays may reclassify part of the occult reservoir, forcing a redefinition of the disease boundary. Standardisation of droplet digital PCR for cccDNA and serum DNA, harmonisation of HBV RNA and core-related antigen measurements, and the incorporation of viral integration mapping into clinical phenotyping are all identified as priorities. As functional cure strategies for chronic hepatitis B, built around HBsAg seroclearance, move into large trials, the review’s central message acquires added urgency: seroclearance is not viral eradication. Every patient whose surface antigen disappears may still harbour replication-competent virus, and the systems of blood banking, transplant medicine, oncology and hepatology must be designed with that hidden reservoir in mind.

Subject of Research: Occult hepatitis B virus infection, the persistence of replication-competent HBV DNA in liver or blood of hepatitis B surface antigen-seronegative individuals

Article Title: Occult hepatitis B virus infection

Article References: Mak, L.-Y., Hui, R. W.-H., Pollicino, T., Cornberg, M., Gish, R., Jacobson, I., Kennedy, P. T., Seto, W.-K., Raimondo, G., & Yuen, M.-F. (2026). Occult hepatitis B virus infection. Nature Reviews Gastroenterology & Hepatology. https://doi.org/10.1038/s41575-026-01257-x

Image Credits: AI Generated

DOI: 10.1038/s41575-026-01257-x

Keywords: occult hepatitis B, hepatitis B virus, HBV DNA, cccDNA, hepatitis B surface antigen, anti-HBc, hepatocellular carcinoma, HBV reactivation, droplet digital PCR, blood transfusion transmission, liver transplantation, nucleoside analogues