Why is the Hepatitis B vaccine split into 3, any what is the blood test for at the end?
The answer to this is really no different than this same question about any other vaccine that is given in multiple doses. I'll quote some good points and sources below but understanding how vaccines work in general first, and then getting information on the others questions will give you the best understanding and answer to your question.
According to the Children's Hospital of Philadelphia Vaccine Education Center, vaccines are made of dead or weakened antigens. They
can't cause an infection, but the immune system still sees them as an
enemy and produces antibodies in response. After the threat has
passed, many of the antibodies will break down, but immune cells
called memory cells remain in the body.(1)
When the body encounters that antigen again, the memory cells produce
antibodies fast and strike down the invader before it's too late.(1)
Why is the Hepatitis B vaccine split into 3 doses?
The human body's immune system (and everyone's is different) needs to program itself at the molecular level to build antibodies for fighting off future [antigens] bugs with similiar biomolecular signatures. Medical studies suggest that it's just more efficient to give people two or three doses to ensure the body builds all it needs to at the immune system level to fight these bugs with matching signatures moving forward rather testing all that have been vaccinated to only revaccinate those found that need more than one dose.
Every vaccine ever created has to take many variables into
consideration, he explained, including the individual pathogen or bug;
how our immune systems respond to it; what parts of the bug can be
used to generate an immune response that is protective in nature; and
also how long that response will last. Because that equation is
notably complex, sometimes a second (or third) dose is a good idea.(2)
"Sometimes, if you take a large group of people with one vaccination
you might expect 90 percent [to be protected]," he said. "But if you
give a second dose, you may get up to 98 percent." Rather than testing
the population to find the 10 percent not protected by the first dose,
"what is probably a more straightforward strategy is just giving two
doses to insure you have that high level of protection," he said.(2)
What is the blood test for at the end?
Essentially, the test afterwards confirms that you indeed have the antibodies in your blood to fight off the Hepatitis B virus so your immune system will know to attack it when it's signature is matched since the vaccine programmed your immune system to do so.
If someone has received the hepatitis B vaccine, then a simple blood
test can tell whether they are protected If they have responded to the
vaccine series, the blood test will show a positive result for the
hepatitis B surface antibody (HBsAb+). It is recommended that all
health care workers and household members or sexual partners of an
infected individual have their antibody levels tested one month after
completing the vaccine series.(3)