Review
The duration of immunity to hepatitis B virus (HBV) from plasma-derived vaccine was generally believed to be around 10 years. However, rigorous determination of the upper time limit for immunity has not been carried out. Such information could be useful in devising vaccination schedules and possibly for developing public health policy with respect to the rationale for and timing of booster vaccinations.

McMahon and colleagues at the Centers for Disease Control and Prevention studied 1578 Alaska Natives vaccinated at age 6 months or older between 1981 and 1982. Subjects received three doses of plasma-derived hepatitis B vaccine appropriate for their age, and were tested annually for antibodies to hepatitis B surface antigen (anti-HBs) and HBV infection markers for the first 11 years.

Anti-HBs levels decreased in the study population from a mean concentration of 822 mIU/mL after vaccination to 27 mIU/mL at 15 years. Higher levels of anti-HBs were noted in males, individuals with higher initial anti-HB levels, and those who were older at the time of vaccination. After adjusting for initial anti-HBs level and sex, the lowest HBs levels at 15 years post-vaccination were observed in those vaccinated between 6 months and 4 years of age.

Researchers detected asymptomatic breakthrough infections in 16 participants. Infection occurred more frequently in vaccine non-responders than in responders (P=.01).

The authors concluded that hepatitis B vaccination protects strongly against infection for at least 15 years.

One limitation of the study was that only slightly more than half of the original cohort had the 15-year follow-up testing.

Commenting in the same issue of Annals of Internal Medicine, Ding-Shinn Chen, National Taiwan University College of Medicine, wrote that before the McMahon study, the duration of protection conferred by hepatitis B vaccine was unknown.

Chen noted McMahon's observation of a precipitous decline in antibodies at 15 years in individuals vaccinated at a very young age. "Therefore, children vaccinated in early childhood may be more likely to have undetectable levels of anti-HBs in late adolescence or early adulthood, when the risks of HBV infection increase because of sexual activity or other social behaviors," Chen wrote.

Loss of anti-HBs does not necessarily mean loss of immunity, as the immunologic memory induced by the HBV vaccine persists even as anti-HBs decline. Further follow-up is therefore needed to determine if and when booster vaccinations are necessary in the general population.