The claim states that immunity can be developed naturally without vaccination through exposure. Multiple papers (e.g., papers 0, 2, 5, and 9) provide clear evidence that natural exposure to pathogens or other stimuli induces adaptive immune responses and trained immunity in humans. None of the retrieved papers refute this biological fact; some compare natural immunity to vaccine-induced immunity (often finding differences in magnitude or duration), but they universally acknowledge the existence of naturally acquired immunity from prior infection or exposure.