Infection with hepatitis B virus (HBV) may lead to acute or chronic hepatitis. HBV infections were previously much more frequent but there are still 240 million chronic HBV carriers today and ca. 620,000 die per year from the late sequelae liver cirrhosis or hepatocellular carcinoma. Hepatitis B was recognized as a disease in ancient times, but its etiologic agent was only recently identified. The first clue in unraveling this mystery was the discovery of an enigmatic serum protein named Australia antigen 50 years ago by Baruch Blumberg. Some years later this was recognized to be the HBV surface antigen (HBsAg). Detection of HBsAg allowed for the first time screening of inapparently infected blood donors for a dangerous pathogen. The need to diagnose clinically silent HBV infections was a strong driving force in the development of modern virus diagnostics. HBsAg was the first infection marker to be assayed with a highly sensitive radio immune assay. HBV itself was among the first viruses to be detected by assay of its DNA genome and IgM antibodies against the HBV core antigen were the first to be selectively detected by the anti-μ capture assay. The cloning and sequencing of the HBV genome in 1978 paved the way to understand the viral life cycle, and allowed development of efficient vaccines and drugs. Today’s hepatitis B vaccine was the first vaccine produced by gene technology. Among the problems that still remain today are the inability to achieve a complete cure of chronic HBV infections, the recognition of occult HBV infections, their potential reactivation and the incomplete protection against escape mutants and heterologous HBV genotypes by HBV vaccines.

The Taormina Consensus Conference defined ‘occult hepatitis B virus (HBV) infection’ (OBI) as the ‘presence of HBV DNA in the liver of individuals testing HBsAg-negative with currently available assays’. Most occult is the so-called ‘window period’ after exposure before HBV DNA appears in the blood. We identified two blood donors whose donations tested HBsAg- and HBV DNA-negative, but transmitted HBV. Both subsequently developed HBsAg and acute hepatitis. However, such cases are not considered as true OBI. A true transient OBI remains HBsAg-negative during the entire course. One case of acute OBI showed a peak viremia of 15,000 IU/ml HBV DNA and sub-borderline HBsAg, suggesting a ratio of virions to subviral particles of 1:10, whereas ‘normal’ cases show at peak viremia a ratio of 1:3,000. Blood donors with OBI may transmit HBV. We studied 5 blood donors with OBI and 55 of their recipients. In 22 recipients, transmission was probable, but they remained healthy. However, in 3 recipients, who were immunosuppressed at the time of transfusion, fatal fulminant hepatitis B developed. The majority of anti-HBc-positive healthy individuals have HBV DNA in the liver which may start replication under severe immunosuppression. Nine such cases are described here. OBI or reactivated HBV infections often lead to selection of HBsAg escape mutations as we could show in 11 of 14 cases. Infection of vaccinated individuals favors development of OBI as we observed in 6 blood donors. HB vaccination may solve the problem of overt HBV infection but may favor OBI.

Potential conflicts of interest. W.G.: no conflicts.