“One of the major arguments against vaccine-induced immunity is that it primarily stimulates the humoral immune system and not the cellular immune system. Antibodies are produced by the humoral immune system and then routinely measured to determine ‘immunity.’
The problem with this approach is that you can have high antibody levels and still get the disease. It’s very difficult and expensive to measure the cellular immune response, and immunologists admit that they are still in the dark about a lot of the finer points of the overall immune response.
When you use antibody titers or blood levels to check for immunity, all you’re doing is getting a picture of what happened (you had an immune response); it doesn’t tell you whether you’re going to be immune in the future, because antibodies are only one aspect of the immune response, and in some cases are not even necessary to easily combat the sickness and become immune.
For example, those with agammaglobulinemia—a disease where you cannot make antibodies—can get infected with measles, recover uneventfully, and still respond to subsequent challenges of the virus in a normal healthy fashion and not get sick. These individuals will have lifelong immunity to measles, the same as someone without agammaglobulinemia.
Traditionally, the way immunity is determined is to do a test that measures antibodies, which is the humoral immune system. But there’s no good way to assess the cellular immune system. It’s a really imprecise science at best.”