Hepatitis Viruses

The five hepatitis viruses are taxonomically unrelated, spanning four entirely different virus families, but are grouped together because they share a single defining clinical target: the liver. Despite this shared tropism, they differ enormously in genome structure, transmission route, chronicity risk, and oncogenic potential, and distinguishing between them on exactly these points is the core clinical task.

Both are nonenveloped, faecal-oral RNA viruses. They cause self-limited acute hepatitis with no meaningful chronic carrier state in immunocompetent hosts. Neither carries cirrhosis or hepatocellular carcinoma risk, which sharply distinguishes this pair from the blood-borne hepatitis B, C, and D group.

Hepatitis E breaks from this otherwise close clinical parallel in two specific ways:

• a markedly elevated mortality risk in pregnancy, particularly the third trimester
• a recognised capacity for chronic infection specifically in immunocompromised hosts, notably solid organ transplant recipients

Hepatitis A shows neither feature.

Genome and structure. A partially double-stranded, circular DNA genome, an unusual structure among DNA viruses that reflects the virus's distinctive replication strategy. Enveloped, belonging to the hepadnavirus family.

Distinctive replication feature. Despite being a DNA virus, HBV replicates via an RNA intermediate. A virus-encoded reverse transcriptase copies this intermediate back into the DNA genome packaged into new virions.

This replication strategy is analogous in principle to retroviruses (reverse transcription). However, HBV does not integrate its genome into the host chromosome as an obligatory part of normal replication.

Occasional random integration does occur and is relevant to its oncogenic mechanism. Unlike in HIV, it is not an essential step in the standard replication cycle.

Transmission.

• Parenteral: blood and blood product exposure, injecting drug use, needlestick injury.
• Sexual contact.
• Vertical (perinatal): from an infected mother to her infant during delivery. This is a highly efficient route and was a major driver of chronic HBV prevalence in historically high-endemicity regions before widespread infant vaccination.

Chronicity, age dependency. The risk of progression from acute to chronic infection is strongly and inversely related to age at acquisition. Perinatal and infant infection carries a very high risk of chronicity, often cited around 90 percent. The risk declines substantially with infection acquired later in childhood, and is comparatively low, around 5 to 10 percent, with infection acquired in healthy immunocompetent adulthood.

This explains why perinatal transmission prevention (maternal screening and infant immunoprophylaxis) is such a central public health priority for this virus.

Cirrhosis and hepatocellular carcinoma risk. Both are significantly elevated with chronic infection. HBV can also cause hepatocellular carcinoma via a direct oncogenic mechanism independent of cirrhosis. In hepatitis C, by contrast, malignancy risk is predominantly, though perhaps not exclusively, mediated via chronic inflammation and cirrhosis.

Vaccine. A highly effective recombinant subunit vaccine targeting HBsAg is available. It is incorporated into routine infant immunisation schedules in most countries globally. It has a direct, measurable impact on acute disease and, over subsequent decades, on the incidence of chronic HBV-related cirrhosis and hepatocellular carcinoma.

HBV serology is more complex than for any other hepatitis virus. The pattern of markers gives precise information about infection stage, chronicity, infectivity, and immune status, so fluent interpretation of the panel is a core clinical skill.

HBsAg, hepatitis B surface antigen, is a structural viral envelope protein. Its presence indicates current infection, either acute or chronic, distinguished by the accompanying serological pattern. It is also the antigen targeted by the vaccine. Persistence beyond six months defines chronic infection.

HBeAg, hepatitis B e antigen, is a secreted, nonstructural viral protein. Its presence correlates with high viral replication and high infectivity, and indicates active, transmissible infection with a high viral load.

Anti-HBs, antibody to surface antigen, indicates immunity. It arises in one of two ways:

• Resolution of natural infection: it typically appears alongside anti-HBc IgG, which allows distinction from vaccine-induced immunity.
• Vaccination: anti-HBc remains negative, since the vaccine contains only HBsAg and never exposes the recipient to core antigen.

Anti-HBs is the marker checked to confirm vaccine-induced protective immunity.

Anti-HBc IgM, antibody to core antigen, IgM class, indicates recent, acute infection. It appears early and persists for several months before declining. It is the key marker for distinguishing a genuinely acute presentation from a chronic infection presenting with an acute-appearing flare.

Anti-HBc IgG, antibody to core antigen, IgG class, indicates past or current infection at any point since acute infection. It persists lifelong, whether the infection resolved or became chronic.

Critically, vaccination alone never produces this marker, since the vaccine contains no core antigen. A positive anti-HBc IgG therefore reliably distinguishes natural infection-derived immunity from vaccine-derived immunity when anti-HBs is positive.

Anti-HBe, antibody to e antigen, typically appears as HBeAg declines. It generally correlates with declining viral replication and reduced infectivity. The transition from HBeAg-positive to anti-HBe-positive is sometimes called seroconversion.

Some chronic HBV variants (precore and core promoter mutants) can maintain high viral replication despite being HBeAg-negative and anti-HBe-positive. This is a specific exception to the general infectivity correlation, which should not be treated as absolute.

Interpreting common patterns.

• Acute infection: HBsAg positive, anti-HBc IgM positive, HBeAg typically positive.
• Chronic infection: HBsAg positive beyond six months, anti-HBc IgG positive, anti-HBc IgM negative, HBeAg status variable depending on replication phase.
• Resolved past infection: HBsAg negative, anti-HBs positive, anti-HBc IgG positive.
• Vaccination-induced immunity: HBsAg negative, anti-HBs positive, anti-HBc negative (both IgM and IgG). This pattern distinguishes vaccination from natural infection resolution.

The window period is an occasionally encountered scenario in which HBsAg has already declined to undetectable but anti-HBs has not yet risen to detectable levels. Anti-HBc IgM is the only positive marker during this interval. This is a diagnostic pitfall when interpreting an apparently isolated anti-HBc IgM-positive result.

Genome and structure. Positive-sense ssRNA, enveloped, a flavivirus. It is family-related to, though clinically entirely distinct from, the arboviral flaviviruses such as dengue and yellow fever.

Transmission. Predominantly parenteral. Injecting drug use is the dominant contemporary route in most high-income settings. Blood transfusion before screening was introduced was a major historical route. Sexual transmission occurs but substantially less efficiently than for hepatitis B or HIV. Vertical transmission occurs at a comparatively lower rate than for hepatitis B.

Chronicity. The majority of infections progress to chronic infection, generally cited around 55 to 85 percent depending on the population and specific study. This is a substantially higher chronicity rate than hepatitis B in adult-acquired infection. Unlike hepatitis B, chronicity risk for hepatitis C does not show a strong age-at-acquisition dependency.

Cirrhosis and hepatocellular carcinoma risk. Both are significantly elevated with chronic infection. Risk is predominantly mediated via chronic inflammation and progressive fibrosis and cirrhosis, rather than the direct oncogenic mechanism seen in hepatitis B, though the distinction is not absolute.

Vaccine. None is available, reflecting the same fundamental challenge of high mutation rate and extensive quasispecies diversity that has frustrated HIV vaccine development.

Distinctively among the viruses in this group, highly effective direct-acting antiviral therapy now achieves durable cure in the large majority of treated patients. This is a different and more optimistic therapeutic picture than for the other chronic viral infections.

Genome and structure. A negative-sense, circular ssRNA genome. HDV is a defective satellite virus, entirely dependent on hepatitis B virus for its envelope. HDV particles are coated in HBsAg borrowed directly from HBV, so HDV cannot infect in the absence of concurrent HBV infection. This dependency is unique among human viruses.

Transmission. The same parenteral, sexual, and vertical routes as HBV, given the obligate co-dependency.

Two distinct infection patterns.

• Co-infection is simultaneous acquisition of HBV and HDV. It generally produces a more severe acute hepatitis than HBV alone. Chronicity risk for the HDV component broadly follows HBV chronicity itself, since HDV cannot persist if the underlying HBV infection resolves.
• Superinfection is HDV infection in a patient with pre-existing chronic HBV infection. It generally produces a more severe acute presentation than co-infection and a substantially higher rate of progression to chronic HDV infection. The underlying chronic HBV infection already provides the ongoing HBsAg supply HDV depends upon.

Cirrhosis risk. The highest of any hepatitis virus in this comparison when superinfecting chronic HBV, reflecting the combined, synergistic liver injury from both viruses.

Vaccine. No dedicated HDV vaccine exists. HBV vaccination indirectly and completely prevents HDV infection, given HDV's absolute dependency on HBV. This is a further benefit of universal HBV vaccination programmes.