One theory is that it's due to the vector. This utilizes an adenovirus (which has been used more commonly in vaccines previously, and is different from the nove

One theory is that it's due to the vector. This utilizes an adenovirus (which has been used more commonly in vaccines previously, and is different from the novel mRNA viruses developed by Biontech/Pfizer/Moderna). The thought process is that with a larger initial dose of the vaccine, more immune resistance is built up by the body initially, which then neutralizes some of the booster when its given later.