Timing of Third Hepatitis B Vaccine Dose After Delayed Second Dose
If the second dose of hepatitis B vaccine was given at 6 months, the third dose should be administered at least 8 weeks after the second dose (at approximately 8 months of age or later), ensuring it is also at least 16 weeks after the first dose. 1, 2
Key Timing Requirements
The Advisory Committee on Immunization Practices (ACIP) establishes clear minimum intervals that must be respected when completing an interrupted hepatitis B vaccine series 1:
- Minimum interval between dose 2 and dose 3: At least 8 weeks 1, 2
- Minimum interval between dose 1 and dose 3: At least 16 weeks 1, 2
- For infants: The final dose cannot be administered before 24 weeks of age 1, 2
Critical Management Principle
The vaccination series should never be restarted when interrupted—simply continue where you left off. 1, 2 This is based on immunologic memory, which remains intact despite delays in the schedule 2.
Practical Application to Your Scenario
Since the second dose was given at 6 months:
- Administer the third dose as soon as possible after meeting the minimum 8-week interval from dose 2 1, 2
- Verify that at least 16 weeks have elapsed since the first dose before giving dose 3 1, 2
- For infants, ensure the child is at least 24 weeks (6 months) of age before administering the final dose 1, 2
Grace Period Rule
Doses administered up to 4 days before the minimum interval are considered valid and do not need to be repeated 1, 2. However, this grace period should not be used routinely—it exists only to accommodate scheduling challenges.
Common Pitfalls to Avoid
- Do not give the third dose before 8 weeks from the second dose, even if 16 weeks have passed since the first dose 1, 2
- Do not restart the series due to the delay—this wastes doses and unnecessarily prolongs the vaccination schedule 1, 2
- For infants, do not administer the final dose before 24 weeks of age, as this may compromise long-term immunity 2
Post-Vaccination Considerations
For high-risk individuals (healthcare workers, hemodialysis patients, HIV-infected persons, immunocompromised individuals), anti-HBs testing should be performed 1-2 months after completing the series to confirm seroprotection (≥10 mIU/mL) 2, 3.