The Injection Problem
Summary: Injections bypass critical components of the immune training system.

Massive amounts of research has gone into the workings of the immune system; yet there are many areas that are still not well understood. One of these is the Mucosa-Associated Lymphatic Tissues (MALT) component, which comprises a very large portion of your immune system, probably at least half. This system has a very special function, responsible for discerning the difference between invaders and helpful compounds such as foods and friendly bacteria.
Without getting too technical, you have a vast number of immunity components positioned in the mucous membranes of your body: along your respiratory system (nose pharynx, lungs), your digestive system (mouth, esophagus, stomach, intestines), and other areas exposed to the external environment such as your eyes, tear ducts, etc.
These components are crucial for processing foreign entities such as germs and food proteins, deciding which are dangerous and which can be allowed to exist in your body. Injection of germ or food particles directly into your body bypasses this processing.
For example, a peanut protein injected before any peanuts are eaten (before they are processed by MALT to recognize them) causes an abnormal sensitivity (a sort of panic reaction) to peanuts that can be very severe, even deadly.
Virus and bacteria parts used to create vaccines, and other vaccine components, create an unnatural reaction when your body doesn't recognize them without the MALT processing. Thus, the results are less predictable. This is one reason why natural immunity is so much superior to vaccination; natural immunity usually lasts a lifetime, while vaccination requires booster shots and generally offers a somewhat less protective effect.
See also:
- Regarding sensitization to injected food proteins versus oral consumption: "The feeding of foreign antigens leads typically to a state of specific and active unresponsiveness, a phenomenon known as oral tolerance. Thus, no antibody response follows the feeding of a foreign protein such as ovalbumin*, although a strong antibody response to this protein can be induced by injecting it subcutaneously, especially if an adjuvant is given as well."
The Mucosal Immune System – Immunobiology: The Immune System in Health and Disease, 5th edition.*Note: You don't want an antibody response to food proteins such as ovalbumin, the protein found in egg whites and contained in some vaccines; that will create an allergic reaction to eggs.