We agree with Dr. Tabor that our findings1 cannot be extrapolated to recipients of other hepatitis B (HB) vaccines, and hence more data are needed to issue a recommendation of routine booster vaccination for recombinant HB vaccine recipients. Although a qualitative difference does exist between plasma-derived and genetically engineered HB vaccine, the durability of antibodies produced by them may not differ from each other. Studies to understand long-term outcome of recipients of recombinant HB vaccine are the priority data that we need.
We also appreciate the comments by Dr. Chan and colleagues on our report. HB vaccine is indeed a reliable vaccine that has prevented chronic infections in most of vaccinees. Many previous reports, including one by ourselves2 showed a routine booster vaccination may not be required to provide protection against chronic HB virus infection before age 15 years. However, most of the studies were confined to subjects in early ages of life. The real threat may emerge when the vaccinated subjects become older and begin to both engage in sexual activity and get along with older generations who still have a high HB surface antigen (HBSAg) carrier rate. Encouragement of safe sex, of course, is certainly an option in preventing not only HB infection but also other sexually transmitted diseases. However, if safe sex practice could be easily executed, diseases such as AIDS would not spread so rampantly. Maintaining and prolonging the protection by vaccination may be a more reliable choice. Nevertheless, we agree that cost-effectiveness analyses are needed.
Our data1 showed that 2.7% of children whose postvaccination anti-HBs had been documented did not respond to one dose of HB vaccine booster at the age of 15 years. Besides, a blunted serological response was noted in approximately 20% of the patients. Furthermore, a breakthrough infection was found in a child whose anti-HBs had been documented after primary immunization and persisted to age 7 years. All the findings imply the immune memory, either humoral or cellular immunity, has waned significantly. A booster dose given prior to age 15 years or a two-dose HB vaccine regimen would have been needed for the children to maintain a protective level of antibody. Although Dr. Chan stated the percentage of patients that became HBsAg-positive was low (1/26, 4%), we are concerned with the possibility that this number will increase as the immune memory continues to decay and due to any possible increased risk of exposure in life after the teenage years. Generally speaking, a booster dose is needed to maintain the protection conferred by a killed vaccine. From the perspective of public health, if a booster vaccination program is to be implemented, it is imperative to set booster vaccination at an age when all the children are still easily accessible and one-dose regimen is universally effective in inducing a protective antibody.
Although we are not yet ready to recommend routine booster vaccination for all HB vaccines, our study on plasma-derived vaccine provided an important observation that for sure will invite more elegant studies to elucidate this issue.