Please join Arnica - Parents' Support Network, Promoting Natural Immunity on Facebook. You will be very confused asking questions here!
There is no such things as, 'promoting natural immunity'. If you are well nourished and in good health you are likely to have a healthy immune system and normal immunity.
You can't 'boost' or increase this (well not naturally, there are a couple of medical treatments that can do it temporarily), which is good because as some one living with an autoimmune disease I would not wish it on my worst enemy.
I refused vaccination while pregnant because there were no studies to prove the safety or benefits of such vaccines; in fact until recently the normal advice was not to vaccinate pregnant women at all.
Err, you do understand the reasons for that don't you?
It's not exactly ethical to do trials on pregnant women. Even if it were there are too many variables.
So why has the advice changed? Well because sometimes people have little choice. If you are pregnant and have a child or are exposed to a child with a disease that can harm a baby before birth or soon after then you would be given the option of vaccinating.
This was why my mother chose to have gamma globulin, no rubella vaccine. And the one that exists now you can't have in pregnancy.
Once a vaccine has been given to a pregnant woman records are updated and if there is an advantage then they become recommended.