Hepatitis A Vaccine Dosing for a 19-Year-Old
A previously unvaccinated 19-year-old should receive a 2-dose series of single-antigen hepatitis A vaccine: either Havrix (1.0 mL containing 1440 ELISA units) at 0 and 6-12 months, or Vaqta (1.0 mL containing 50 units) at 0 and 6-18 months. 1, 2
Standard Dosing Schedules
The ACIP guidelines provide clear dosing parameters for adults aged 19 years and older:
- Havrix: 1.0 mL (1440 ELISA units) administered intramuscularly, with the second dose given 6-12 months after the first dose 1, 2
- Vaqta: 1.0 mL (50 units of hepatitis A viral antigen) administered intramuscularly, with the second dose given 6-18 months after the first dose 1, 2
Alternative Combination Vaccine Option
If combined hepatitis A and B protection is desired, Twinrix can be used with two scheduling options:
- Standard schedule: 3 doses at 0,1, and 6 months (1.0 mL per dose containing 20 μg hepatitis B antigen) 1, 2
- Accelerated schedule: 4 doses at days 0,7,21-30, followed by a booster at 12 months 1, 2
Immunogenicity and Protection Timeline
The evidence demonstrates robust immune response with single-antigen vaccines:
- Rapid seroconversion: 88-91% of recipients achieve protective antibody levels within 14-15 days after a single dose 3, 4
- One-month seroconversion: Reaches 99-100% by one month after the initial dose 3, 4
- Pre-booster persistence: 97% maintain detectable antibodies at 6 months before the booster dose 4
- Post-booster response: All vaccinees demonstrate measurable titers after the second dose, with antibody levels rising 73-fold higher than peak levels seen with immune globulin 5
Important Clinical Considerations
Vaccination indications for this age group include anyone seeking protection from hepatitis A, plus specific risk groups 1:
- Men who have sex with men
- Persons who use injection or noninjection illicit drugs
- Travelers to countries with high or intermediate hepatitis A endemicity
- Persons with chronic liver disease
- Persons working with HAV in laboratory settings
- Homeless individuals 1
Schedule flexibility: If the vaccination series is interrupted, it does not need to be restarted—simply administer the second dose as soon as possible 1, 6. The minimum interval between doses should be maintained (6 months for Havrix, 6 months for Vaqta), but there is no maximum interval 1.
Common pitfall to avoid: Do not use pediatric dosing (0.5 mL) in adults aged 19 years or older, as this provides suboptimal antigen content (720 ELISA units for Havrix or 25 units for Vaqta) 7. Adults require the full 1.0 mL adult formulation for adequate immunogenicity 1, 7.
Pregnancy considerations: Both Havrix and Vaqta are safe for use in pregnancy if indicated 7. If using Twinrix, note that Heplisav-B component data are insufficient for pregnancy, so single-antigen vaccines may be preferred in pregnant women 1.