Introduction

Hepatitis B virus (HBV) is an enveloped, hepatotropic, partially double-stranded DNA virus belonging to the family Hepadnaviridae.

HBV is one of the most important human viral pathogens because infection may produce:

• Acute hepatitis

• Fulminant hepatitis

• Chronic hepatitis

• An inactive infection state

• Cirrhosis

• Hepatic decompensation

• Hepatocellular carcinoma (HCC)

• Extrahepatic manifestations

Unlike many other DNA viruses, HBV replicates through an RNA intermediate and reverse transcription, making it a particularly important virus from a molecular microbiology perspective.

The ability of HBV to establish a stable nuclear covalently closed circular DNA (cccDNA) reservoir within hepatocytes is central to the persistence of infection and the difficulty of achieving a definitive cure.

1. Classification

Virus

Hepatitis B virus (HBV)

Family

Hepadnaviridae

Genus

Orthohepadnavirus

Genome

• DNA

• Circular

• Partially double-stranded

• Approximately 3.2 kb

Envelope

Present

Replication strategy

Reverse transcription of an RNA intermediate

Principal target cell

Hepatocyte

Major site of replication

Liver

2. Why HBV Is Unique

HBV is unusual because it is a DNA virus that uses reverse transcriptase.

The simplified replication concept is:

DNA → RNA → DNA

More specifically:

cccDNA → viral RNA → pregenomic RNA → reverse transcription → relaxed circular DNA (rcDNA)

This makes HBV mechanistically different from most conventional DNA viruses.

The virus also maintains a nuclear cccDNA reservoir that can persist even when circulating HBV DNA becomes undetectable.

3. Virion

The complete infectious HBV virion is called the:

Dane particle

The Dane particle is approximately 42 nm in diameter.

It consists of:

1. Lipid envelope

2. Surface proteins

3. Nucleocapsid/core

4. Viral DNA

5. Viral DNA polymerase/reverse transcriptase

The envelope contains the three forms of hepatitis B surface protein:

• Large HBs protein — L-HBs

• Middle HBs protein — M-HBs

• Small/major HBs protein — S-HBs

The nucleocapsid contains:

• HBcAg

• Viral genome

• Viral polymerase

4. Subviral Particles

An important characteristic of HBV is the production of enormous quantities of non-infectious subviral particles.

These occur mainly as:

1. Spherical particles

Approximately 20–22 nm.

2. Filamentous particles

Approximately 20 nm in diameter but variable in length.

They contain HBsAg but do not contain the complete viral genome and therefore are non-infectious.

These particles greatly outnumber complete Dane particles.

Why is this important?

The massive production of HBsAg-containing particles has several consequences:

• HBsAg becomes readily detectable in serum.

• The amount of circulating HBsAg greatly exceeds the amount incorporated into infectious virions.

• These particles may act as an immunological decoy by absorbing anti-HBs antibodies.

5. Genome

HBV contains a small, highly compact genome of approximately 3.2 kb.

It is:

• Circular

• Partially double-stranded

• Relaxed circular DNA (rcDNA) in the mature virion

The genome contains four major overlapping open reading frames:

1. S

2. C

3. P

4. X

These encode the major viral proteins.

S gene

Encodes:

• Large surface protein

• Middle surface protein

• Small surface protein

C/pre-C gene

Encodes:

• Core protein

• HBeAg-related precore product

P gene

Encodes:

• Viral polymerase/reverse transcriptase

X gene

Encodes:

• HBx protein

The extreme compactness of the genome is achieved through extensive overlap between ORFs.

6. Major HBV Antigens

The major antigens relevant to clinical microbiology are:

HBsAg

Hepatitis B surface antigen

Indicates current HBV infection when appropriately interpreted.

It is the principal screening marker for HBV infection.

HBcAg

Hepatitis B core antigen

Located within the nucleocapsid.

HBcAg is not normally detectable as a free circulating antigen in routine serum testing.

However, the immune response against HBcAg produces:

• Anti-HBc IgM

• Anti-HBc IgG/total anti-HBc

These antibodies are extremely important for determining the stage and nature of infection.

HBeAg

Hepatitis B e antigen

HBeAg is a secreted viral protein derived from the precore/core region.

Historically, HBeAg has been used as a marker of active viral replication and increased infectivity.

However:

This is particularly important because precore and basal core promoter mutations can produce HBeAg-negative HBV infection with ongoing HBV replication.

7. HBV Surface Proteins

Three major envelope proteins are produced from the S region.

The pre-S1 region of the large surface protein plays an important role in hepatocyte attachment and entry.

A critical host receptor involved in HBV entry is:

NTCP

Sodium taurocholate cotransporting polypeptide

The myristoylated pre-S1 domain of L-HBs interacts with NTCP on hepatocytes.

8. HBV Replication — The Central Concept

HBV replication is one of the most important concepts in medical virology.

The simplified sequence is:

Attachment → Entry → Nuclear transport → cccDNA formation → Transcription → Translation → Encapsidation → Reverse transcription → rcDNA formation → Envelopment → Release

Step 1 — Attachment

HBV reaches the liver through the bloodstream.

The viral envelope interacts with hepatocyte surface molecules.

The pre-S1 region of the large surface protein is particularly important for receptor interaction.

Step 2 — Entry

HBV enters hepatocytes by receptor-mediated mechanisms involving NTCP.

Step 3 — Nuclear Transport

Following entry, the nucleocapsid is transported toward the nucleus.

The viral relaxed circular DNA eventually reaches the nucleus.

Step 4 — Formation of cccDNA

The incoming relaxed circular DNA is repaired within the nucleus to produce:

Covalently closed circular DNA

cccDNA

This is one of the most important biological features of HBV.

cccDNA behaves as a stable episomal transcriptional template within the hepatocyte nucleus.

9. Transcription

Host cellular RNA polymerase II transcribes the cccDNA template.

Multiple viral RNAs are produced.

The most important is:

Pregenomic RNA — pgRNA

The pgRNA serves two functions:

1. Template for viral protein synthesis

2. Template for reverse transcription into viral DNA

This dual role is fundamental to HBV biology.

10. Translation

Viral RNAs are translated to produce:

• Surface proteins

• Core protein

• Polymerase

• X protein

• Precore/HBeAg-related products

11. Encapsidation

The pgRNA and viral polymerase are selectively packaged into newly forming nucleocapsids.

The polymerase binds the pgRNA through the viral packaging signal.

12. Reverse Transcription

Inside the nucleocapsid:

pgRNA → minus-strand DNA → plus-strand DNA

The viral polymerase possesses reverse transcriptase activity.

The RNA template is progressively degraded by the RNase H activity of the polymerase.

Because plus-strand DNA synthesis is incomplete when the virion is released, the mature genome remains:

Partially double-stranded relaxed circular DNA

13. Virion Assembly and Release

Newly formed nucleocapsids have two possible fates:

Pathway 1

Return to the nucleus and replenish the cccDNA pool.

Pathway 2

Acquire an envelope containing HBs proteins and leave the hepatocyte as mature virions.

This dynamic allows HBV to maintain persistent intracellular infection.

14. Pathogenesis

A critical principle is:

The severity of liver injury therefore depends greatly on the interaction between:

• Viral replication

• Viral antigens

• Innate immunity

• Adaptive immunity

• CD8+ T-cell response

• CD4+ T-cell help

• Cytokine responses

• Host factors

• Viral factors

15. Role of CD8+ T Cells

Virus-specific cytotoxic T lymphocytes recognize HBV-derived peptides presented on:

MHC class I

on infected hepatocytes.

They can destroy infected hepatocytes.

This contributes to:

• Hepatocyte necrosis

• ALT/AST elevation

• Hepatitis

• Clearance of HBV

Thus, the immune response is both:

Protective

because it helps eliminate infected cells and control viral replication.

Pathogenic

because immune-mediated hepatocyte destruction produces liver injury.

16. Why Some Patients Clear HBV and Others Become Chronic

The age at infection is a major determinant.

Perinatal infection

Has a very high probability of progressing to chronic infection.

Early childhood infection

Has an intermediate-to-high risk of chronicity.

Adult infection

Most immunocompetent adults successfully clear acute infection.

Therefore:

Current CDC clinical guidance estimates that approximately 90% of infected infants and approximately 30% of children infected at 1–5 years develop chronic infection, whereas most immunocompetent adults clear acute infection.

17. HBV and Chronic Liver Disease

Persistent HBV infection may produce a continuum:

HBV infection → chronic hepatitis → fibrosis → cirrhosis → hepatic failure/HCC

However, progression is not inevitable.

Patients can pass through different virological and immunological phases characterized by differences in:

• HBeAg

• HBV DNA

• ALT

• Degree of liver inflammation

• Fibrosis

• Immune activity

This is why a single HBV DNA or ALT measurement is insufficient to fully characterize chronic HBV infection.

18. Hepatocellular Carcinoma

HBV is a major cause of hepatocellular carcinoma.

HBV-associated HCC may develop:

• In the presence of cirrhosis

• Sometimes even without established cirrhosis

Mechanisms contributing to carcinogenesis include:

1. Chronic inflammation

Repeated cycles of:

cell injury → regeneration → mutation → clonal expansion

increase carcinogenic risk.

2. Viral DNA integration

HBV DNA may integrate into the host genome.

Although integrated HBV DNA is generally not the productive template for new virions, integration can contribute to genomic instability and altered cellular gene expression.

3. HBx protein

HBx participates in multiple cellular regulatory pathways and may contribute to:

• Transcriptional dysregulation

• Cell-cycle alterations

• DNA damage responses

• Apoptotic regulation

• Signaling abnormalities

4. Chronic immune-mediated injury

Persistent inflammation promotes fibrosis and carcinogenesis.

19. Extrahepatic Manifestations

HBV is not exclusively a liver disease.

Extrahepatic manifestations include:

Immune-complex mediated disease

• Polyarteritis nodosa

• Glomerulonephritis

• Arthralgia/arthritis

• Serum sickness-like manifestations

The underlying mechanism involves circulating immune complexes containing viral antigens and antibodies.

20. Modes of Transmission

HBV is transmitted through exposure to infected blood or certain body fluids.

Major routes include:

1. Perinatal transmission

Mother → infant around the time of birth.

This is particularly important because infection acquired at birth has a high probability of chronicity.

2. Sexual transmission

HBV can be transmitted through sexual exposure to infected body fluids.

3. Percutaneous transmission

Examples include:

• Contaminated needles

• Injection equipment

• Needlestick injuries

• Unsafe injections

• Contaminated medical equipment

• Tattooing/piercing with inadequately sterilized equipment

4. Household transmission

Particularly in high-endemicity settings, transmission may occur through exposure to infected blood or body fluids.

Items such as:

• Razors

• Toothbrushes

may pose a risk if contaminated with infected blood.

5. Healthcare-associated transmission

Healthcare workers are at risk through:

• Needlestick injuries

• Sharps injuries

• Blood exposure

• Inadequately sterilized instruments

21. What Does NOT Normally Transmit HBV?

HBV is not efficiently transmitted by ordinary casual contact.

It is not normally transmitted through:

• Hugging

• Handshakes

• Coughing

• Sneezing

• Sharing food

• Sharing drinking utensils

• Ordinary social interaction

Breastfeeding itself is not considered a reason to withhold breastfeeding when appropriate measures for prevention of mother-to-child transmission have been implemented.

22. Incubation Period

HBV has a relatively long incubation period.

Approximately:

60–90 days is common

but the incubation period can extend considerably depending on the definition and clinical endpoint used.

Clinical symptoms may appear approximately:

60–150 days after exposure

in symptomatic acute infection.

23. Clinical Spectrum

HBV infection can present as:

Asymptomatic infection

Very common, especially in children.

Acute hepatitis

May cause:

• Fever

• Malaise

• Anorexia

• Nausea

• Vomiting

• Right upper quadrant discomfort

• Arthralgia

• Dark urine

• Pale stools

• Jaundice

Fulminant hepatitis

Rare but potentially fatal.

Characterized by severe acute liver injury with hepatic failure and encephalopathy.

Chronic infection

May remain asymptomatic for years or decades.

Cirrhosis

May develop after prolonged inflammatory liver injury.

Hepatocellular carcinoma

May occur with or without established cirrhosis.

24. Acute vs Chronic HBV

Persistence of HBsAg for ≥6 months is the classical laboratory definition of chronic HBV infection.

25. HBV Serology

The major serological markers are:

HBsAg

Indicates current infection.

Anti-HBs

Indicates immunity following:

• Recovery from natural infection, or

• Vaccination

Anti-HBc

Indicates previous or current natural infection.

It is not produced by vaccination alone.

It has two major components:

• IgM anti-HBc

• IgG/total anti-HBc

HBeAg

Generally associated with active viral replication and increased infectivity.

Anti-HBe

Usually indicates seroconversion from HBeAg positivity, although interpretation depends on HBV DNA and disease phase.

26. The Most Important Serological Rule

Vaccination

Produces:

Anti-HBs

but not:

Anti-HBc

Therefore:

whereas:

This distinction is one of the most important principles in HBV serology.

27. HBV DNA

HBV DNA is the direct molecular marker of viral replication.

It is measured using nucleic acid amplification techniques, particularly:

Real-time PCR

HBV DNA is reported commonly as:

• IU/mL

and sometimes logarithmically:

• log10 IU/mL

HBV DNA is essential for:

• Assessing viral replication

• Determining treatment eligibility

• Monitoring treatment response

• Detecting virological breakthrough

• Assessing resistance

• Monitoring patients with chronic HBV

A detailed discussion belongs in Topic 200: HBV Viral Load.

28. Diagnosis

Diagnosis is based on a combination of:

Serology

• HBsAg

• Anti-HBs

• Total anti-HBc

• IgM anti-HBc

• HBeAg

• Anti-HBe

Molecular testing

• HBV DNA

Biochemistry

• ALT

• AST

• Bilirubin

• Albumin

• INR

Liver assessment

Depending on clinical context:

• Ultrasound

• Elastography

• APRI

• FIB-4

• Other non-invasive fibrosis assessment

• Liver biopsy in selected situations

The complete diagnostic algorithm will be covered separately under Topic 199: HBV Diagnosis.

29. Treatment — Basic Concept

There is currently no routinely used therapy that reliably eliminates the nuclear cccDNA reservoir from every infected hepatocyte.

Therefore, treatment aims primarily to:

• Suppress HBV replication

• Reduce hepatic inflammation

• Prevent progression to cirrhosis

• Reduce risk of hepatocellular carcinoma

• Reduce liver-related mortality

• Prevent transmission in appropriate situations

For chronic HBV, major antiviral options include high-potency nucleos(t)ide analogues such as:

• Tenofovir disoproxil fumarate (TDF)

• Tenofovir alafenamide (TAF)

• Entecavir

Pegylated interferon has a role in selected patients.

Current WHO guidance has expanded and simplified eligibility for treatment compared with earlier frameworks.

30. Prevention

HBV is one of the most important vaccine-preventable viral infections.

Prevention includes:

Vaccination

The cornerstone of prevention.

Birth-dose vaccination

Timely administration after birth is particularly important for prevention of mother-to-child transmission.

Screening

Important populations include:

• Pregnant women

• Blood donors

• Healthcare workers

• Household contacts

• Sexual contacts

• People at increased risk of blood exposure

• People from high-endemicity regions

Infection-control measures

• Safe injection practices

• Proper sterilization

• Sharps precautions

• Safe blood screening

• Appropriate PPE

• Needle-stick prevention

Post-exposure prophylaxis

May involve:

• Hepatitis B vaccine

• Hepatitis B immunoglobulin (HBIG)

depending on the exposure and vaccination/serological status of the exposed individual.

31. HBV Vaccine

HBV vaccines are recombinant subunit vaccines containing purified hepatitis B surface antigen.

They do not contain live HBV.

The vaccine induces:

Anti-HBs antibodies

which provide protection against HBV infection.

The vaccine does not contain:

• HBcAg

• Complete infectious virions

• HBV DNA capable of causing infection

Therefore vaccination cannot cause hepatitis B infection.

The detailed discussion of:

• Vaccine types

• Recombinant production

• Yeast expression systems

• Dose

• Schedule

• Birth dose

• Adult vaccination

• Healthcare workers

• Non-responders

• Anti-HBs testing

• Boosters

• HBIG

• Post-exposure prophylaxis

will be covered in Topic 204: HBV Vaccines.

32. HBV Mutants

HBV has substantial genetic variability because reverse transcription is relatively error-prone.

Important clinically relevant mutant categories include:

Precore mutants

May reduce or abolish HBeAg production.

Consequently:

HBeAg negative ≠ HBV DNA negative

Basal core promoter mutants

May alter HBeAg expression and viral replication characteristics.

Surface gene mutants

May alter antigenicity of HBsAg and potentially affect:

• Diagnostic detection

• Vaccine-induced antibody recognition

• Immune escape

Polymerase mutants

May produce resistance to nucleos(t)ide analogues.

Because the polymerase and surface genes overlap extensively, mutations selected in the polymerase region may sometimes influence surface protein sequences as well.

These subjects will be covered individually under Topic 198: HBV Mutants and Topic 201: HBV Drug Resistance.

33. HBV Drug Resistance

Resistance is particularly important with older antiviral drugs having a lower genetic barrier to resistance.

Examples include:

• Lamivudine

• Adefovir

• Telbivudine

High-potency agents with a high barrier to resistance, particularly:

• Tenofovir

• Entecavir in appropriate treatment-naïve settings

are preferred in modern treatment strategies.

Drug resistance may manifest as:

Virological breakthrough

A significant rise in HBV DNA after an initial virological response.

Resistance testing may involve sequencing of the polymerase gene when clinically indicated.

34. HBV and Pregnancy

HBV in pregnancy has two major implications:

Maternal disease

The pregnant patient must be assessed for:

• HBV DNA

• ALT

• Liver disease severity

• Treatment indications

Prevention of mother-to-child transmission

The newborn should receive appropriate prophylaxis promptly after birth.

For mothers with high viral loads, antiviral prophylaxis during pregnancy may be recommended according to current guidelines.

WHO's 2024 guideline specifically expanded eligibility for antiviral prophylaxis during pregnancy to help prevent mother-to-child transmission.

35. HBV and Hepatitis D Virus

HBV is biologically important for another reason:

HDV requires HBV surface antigen.

Hepatitis D virus cannot establish a complete infectious cycle without HBV.

Therefore prevention of HBV through vaccination also prevents hepatitis D infection.

Patients with chronic HBV should therefore be assessed for HDV according to current testing recommendations and clinical context.

36. Natural History — The Big Picture

A useful conceptual model is:

Exposure

↓

Acute HBV infection

↓

Possible outcomes

Immune-mediated clearance

or

Persistent infection

↓

Chronic HBV infection

↓

Possible phases of chronic infection

↓

Chronic hepatitis

↓

Fibrosis

↓

Cirrhosis

↓

Hepatic decompensation / HCC

Importantly, this progression is not inevitable and varies substantially between individuals.

37. High-Yield Microbiology Facts

1.

HBV is a:

DNA virus

but replicates using:

Reverse transcription

2.

HBV belongs to:

Hepadnaviridae

3.

Complete infectious virion:

Dane particle

4.

Approximate genome size:

3.2 kb

5.

Genome:

Partially double-stranded circular DNA

6.

Nuclear replication template:

cccDNA

7.

Important receptor:

NTCP

8.

Major viral antigens:

HBsAg, HBcAg and HBeAg

9.

HBcAg is generally:

Not detectable as free antigen in routine serum

10.

HBeAg generally indicates:

Active replication/increased infectivity, but must be interpreted with HBV DNA.

11.

Anti-HBc indicates:

Natural exposure/infection

It is not produced by vaccination alone.

12.

Anti-HBs indicates:

Immunity

13.

HBsAg persisting ≥6 months:

Chronic HBV infection

14.

Most important cause of liver injury:

Host immune response rather than direct viral cytotoxicity

15.

Important complications:

Cirrhosis + hepatocellular carcinoma

16.

Important extrahepatic manifestations:

Polyarteritis nodosa + glomerulonephritis

17.

Major mechanism of replication:

Reverse transcription of pregenomic RNA

18.

HBV vaccine contains:

Recombinant HBsAg

19.

Vaccination produces:

Anti-HBs

not anti-HBc.

20.

HBV prevention also protects against:

HDV

because HDV requires HBV for its infectious life cycle.

38. One-Minute HBV Revision

Remember HBV as:

Structure

Enveloped + Dane particle + nucleocapsid

Genome

3.2-kb partially dsDNA

Genes

S + C + P + X

Replication

cccDNA → pgRNA → reverse transcription → rcDNA

Main antigens

HBsAg + HBcAg + HBeAg

Main antibody markers

Anti-HBs + anti-HBc + anti-HBe

Disease

Acute → chronic → cirrhosis/HCC

Key pathogenic mechanism

Immune-mediated hepatocyte injury

Diagnosis

Serology + HBV DNA

Treatment

Potent nucleos(t)ide analogues ± pegylated interferon in selected patients

Prevention

Vaccination + safe practices + prevention of mother-to-child transmission

39. HBV — Core Concept Map

40. Take-Home Message

HBV is a small, enveloped, hepatotropic DNA virus with a remarkably sophisticated replication strategy.

Its clinical importance comes from the ability to establish persistent infection through nuclear cccDNA, while its molecular biology is distinguished by reverse transcription of an RNA intermediate.

The clinical outcome is determined by the interaction between:

Virus + host immunity + age at infection + viral replication + duration of infection

The most important long-term consequences are:

Chronic hepatitis → cirrhosis → hepatic failure and/or hepatocellular carcinoma.

Fortunately, HBV is also one of the major viral infections for which highly effective prevention is available through vaccination, together with strategies for preventing mother-to-child transmission and blood/sexual/percutaneous transmission.

Standard References

1. Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. Elsevier.

2. Brooks GF, Carroll KC, Butel JS, Morse SA, Mietzner TA. Jawetz, Melnick & Adelberg's Medical Microbiology. McGraw-Hill.

3. Ryan KJ, Ray CG. Sherris Medical Microbiology. McGraw-Hill.

4. Bennett JE, Dolin R, Blaser MJ, editors. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Elsevier.

5. Fields BN, Knipe DM, Howley PM, editors. Fields Virology. Wolters Kluwer.

6. Flint SJ, Enquist LW, Racaniello VR, Rall GF. Principles of Virology. ASM Press.

7. MacLachlan NJ, Dubovi EJ. Fenner's Veterinary Virology. Elsevier — useful for comparative hepadnavirus biology.

8. World Health Organization. Guidelines for the prevention, diagnosis, care and treatment for people with chronic hepatitis B infection. WHO; 2024.

9. World Health Organization. Hepatitis B Fact Sheet. Updated 28 July 2026.

10. Centers for Disease Control and Prevention. Clinical Overview of Hepatitis B. Updated 2025.

11. Centers for Disease Control and Prevention. Hepatitis B Basics. Updated 2025.

12. European Association for the Study of the Liver. EASL Clinical Practice Guidelines on the management of hepatitis B virus infection.

13. Terrault NA, Lok ASF, McMahon BJ, et al. Update on prevention, diagnosis, and treatment of chronic hepatitis B: AASLD guidance.

14. Knipe DM, Howley PM, editors. Fields Virology: DNA Viruses. Wolters Kluwer.

15. IARC Working Group. Hepatitis Viruses. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. International Agency for Research on Cancer.