Hi Mr Kon
Hepatitis B vaccination uses dead or inactivated Hepatitis B viruses to trigger the immune system to mount a response against it, and in doing so, producing immunity against the virus. It triggers two types of responses:
1. Humeral Response (measured by the antibody level) as well as the
2. Cellular Response (not measurable with any blood tests).
When a vaccine is introduced, most patient will mount a response and produce antibodies. Slowly, the antibody levels will decrease (usual recommendation is to check the levels after 5 years).
When the 5 years is up, and if the antibody level has dropped to below the acceptable levels (usually 10 mIU/mL), then you'll need a single booster dose to boost up your antibody levels again. There is no reason for the decline in antibody levels, it just naturally goes down over time.
In recent years, there are many specialists who believe that even though the antibody levels dropped to zero, the Cellular Response will remain for life, and you will not need a booster dose, as long as you had proven to have a positive antibody level at a certain point in your life.
These specialists will not recommend a booster dose even if your antibody levels have dropped to zero. Most doctors in Singapore will recommend a booster dose though.
However, in your question, you did not specify how long was the antibody test taken after the initial vaccination, and was there ever a positive antibody response for your children.
IF there was never a positive antibody test, we have to consider another possibility of your children being a non-responder, or in the worse case scenario, there is a very small possibility of being infected with a rare Pre-S mutant strain of the Hepatitis B virus. (This is a very rare scenario, and hence most doctors will not scare their patients unnecessarily, but since this platform is the internet, I have to list out possible scenarios).
To fully access the possibilities, kindly see your doctor for further assessment and testing, or you can consult a hepatologist (liver specialist)
Cheers!
