Hello, How are you? Thanks for writing into HCM. My name is Dr Suresh Raghavaiah. I am a liver and pancreas surgeon and I hope to answer your question today. I am sorry to hear about your diagnosis and I am happy to hear that your girlfriend is planning on getting vaccinated. The reason why booster shots are given during vaccination is to create a subset of immune cells called memory B cells . These cells store the memory of the antigen (in this case hepatitis B virus) and will produce protective antibodies as soon as they encounter it the next time (for ex if your girlfriend by any chance gets infected). The creation of memory b cells may require multiple exposures to the antigen - hence the booster doses. Having said that, this holds true to the majority of cases. Some people can develop high levels of protective antibodies even with minute exposure to antigens. The only way to know for sure whether a protective level of antibodies have been attained is to measure the Anti-HbsAG antibody levels (which is done routinely in most laboratories). Some people who are poor responders may need upto 5-6 booster shots before a protective level is reached. So in short, if you want to be sure of the immunity of your girlfriend, I would measure her blood levels of protective antibodies ( 10 IU is considered protective). This does not mean I am discouraging you from getting the booster doses - The only way to be fully immune is to complete the vaccination schedule. Hope this helped and hope you get better. Please do not hesitate to contact me for any further details. Have a great day Dr Suresh Raghavaiah.