Does the Hepatitis B (HepB) vaccine induce an antigen or antibody response?

Hepatitis B Vaccine Induces an Antibody Response

The Hepatitis B (HepB) vaccine contains hepatitis B surface antigen (HBsAg) and induces an antibody response (anti-HBs) in the recipient, not an antigen response. 1

Mechanism of Action

The HepB vaccine works by introducing purified HBsAg protein into the body, which then stimulates the immune system to produce antibodies against this antigen (anti-HBs). This process creates immunological protection through:

1. Vaccine composition:

   • Current HepB vaccines in the US are produced using recombinant DNA technology
   • They contain purified HBsAg protein (10-40 μg/mL) adsorbed to aluminum hydroxide 1
   • The antigen is synthesized by genetically modified yeast (Saccharomyces cerevisiae) 1

2. Immune response:

   • The vaccine triggers production of anti-HBs antibodies
   • Protective immunity is defined as anti-HBs levels ≥10 mIU/mL 1
   • This antibody response provides protection against both acute disease and chronic HBV infection 1

Effectiveness of Antibody Response

The HepB vaccine is highly effective at inducing protective antibody levels:

• Healthy infants: ~95% develop protective antibody levels after completing the 3-dose series 1
• **Healthy adults <40 years**: >90% develop protective antibody levels after the complete series 1
• Progression of response:
  • After first dose: 25-30% of infants and 30-55% of adults develop protective levels
  • After second dose: 63% of infants and 75% of adults develop protective levels
  • After third dose: >95% of recipients achieve protective levels 1

Duration of Protection

The antibody response generated by the HepB vaccine provides long-lasting protection:

• Anti-HBs levels decline over time, particularly in the first year after vaccination 1
• By 18 years post-vaccination, only about 16% of those vaccinated before age 1 maintain antibody levels ≥10 mIU/mL, compared to 74% for those vaccinated at age ≥1 year 1
• Despite declining antibody levels, immunological memory persists for at least 20-30 years in most immunocompetent individuals 1, 2
• When exposed to HBV or given a booster dose years later, most vaccinees (74-100%) demonstrate an anamnestic response with rapid increase in anti-HBs levels 1, 2, 3

Clinical Implications

Understanding that the HepB vaccine induces an antibody response has important clinical implications:

• Protection mechanism: The vaccine-induced anti-HBs antibodies neutralize HBV particles, preventing infection 1
• Testing for immunity: Anti-HBs levels are measured to determine vaccine response and immunity status 1
• Booster considerations: Most immunocompetent individuals don't require booster doses due to persistent immune memory, even when antibody levels fall below 10 mIU/mL 1
• Special populations: Immunocompromised individuals (e.g., dialysis patients, HIV-infected persons) may have reduced antibody responses and might benefit from modified dosing regimens 1

Common Pitfalls

• Confusing terminology: Don't confuse the vaccine (which contains HBsAg) with the antibody response it generates (anti-HBs)
• Misinterpreting low antibody levels: Declining anti-HBs levels over time don't necessarily indicate loss of protection due to persistent immune memory 1
• Overlooking exposure evidence: In vaccinated healthcare workers with occupational exposure to HBV, T-cell responses to HBV core and polymerase antigens may be more sensitive indicators of HBV exposure than antibody tests 4

The HepB vaccine's ability to induce a robust antibody response is key to its effectiveness in preventing HBV infection, with protection lasting for decades in most immunocompetent individuals.