The immune system isn't a container that fills up. It's made of billions of cells that recognise and remember new threats every day.
Environmental antigens far outnumber vaccine antigens
From the moment a person is born, breathing, eating and touching things means constant contact with bacteria and viruses. These microbes carry large numbers of antigens, the markers the immune system uses to identify invaders. Modern vaccines are purified and contain very few antigens per dose. Even several vaccines given on the same day add up to a small fraction of what the body handles naturally every day.
How a vaccine works
A vaccine introduces a weakened or inactivated pathogen, or a small part of it such as a protein, into the body. The immune system learns to make antibodies against it and stores that memory. The next time it meets the real pathogen, the body responds faster and more effectively. This is training, not depletion.
What the research shows
Studies comparing children who received more vaccines with those who received fewer found no increased risk of other infections later, and no evidence that vaccines weaken the immune system overall.
The Centre for Health Protection states that the vaccines under the Hong Kong Childhood Immunisation Programme are safe and effective, supported by long-term evidence.