Abstract Background Current safety frameworks for pediatric vaccines evaluate excipients and co-administered agents independently and in isolation. This approach may be structurally insufficient to detect synergistic effects arising from the simultaneous exposure to multiple agents during critical windows of neonatal neurodevelopment. Existing epidemiological study designs lack the granularity to capture combined exposure, individual homeostatic vulnerability, and the specific clinical phenotype of regressive autism as a distinct outcome separable from the autism spectrum as a whole. Hypothesis We propose that three chemically distinct agents routinely co-administered in the perinatal period converge on a common molecular effector of blood-brain barrier (BBB) permeability: matrix metalloproteinase-9 (MMP-9). The three agents are: (i) aluminum oxyhydroxide adjuvant (Al(OH) 3 ), phagocytosed by macrophages at the injection site and transported biopersistently into the systemic circulation and potentially the CNS; (ii) sorbitol administered as a vaccine excipient by the intramuscular route, reaching the systemic circulation directly without first-pass hepatic metabolism; and (iii) acetaminophen administered for post-vaccination fever, generating the reactive metabolite NAPQI via CYP2E1 activity in astrocytes. Each agent activates MMP-9 through an independent upstream pathway, and all three converge on a secondary node of perivascular glutathione depletion that amplifies barrier vulnerability in the neonatal brain. Mechanistic pathways Al(OH) 3 particles activate the NLRP3 inflammasome in macrophages, driving IL-1β-mediated Th17 differentiation and IL-17A signalling at the cerebrovascular endothelium with direct upregulation of MMP-9. Intramuscularly injected sorbitol enters systemic circulation intact and activates the polyol pathway in perivascular endothelial cells, generating advanced glycation end-products (AGEs) that signal via RAGE to induce oxidative stress and microglial MMP-9 release. Acetaminophen-derived NAPQI depletes astrocytic glutathione locally, impairing the antioxidant support that perivascular astrocytic endfeet provide to the BBB endothelium and driving microglial pro-inflammatory polarization with secondary MMP-9 secretion. The co-administration of all three agents during the same perinatal window has not been tested as a system in any preclinical or clinical study. Conclusion This hypothesis does not assert causality between vaccination and regressive autism. It identifies a mechanistic gap in current regulatory toxicology: a triple convergence on MMP-9 that existing safety frameworks have not tested and current epidemiological designs cannot detect. The hypothesis is falsifiable and warrants prospective mechanistic and epidemiological investigation.
Añaños et al. (Thu,) studied this question.
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