LNP composition and tissue distribution. Tozinameran (BNT162b2/Comirnaty) is administered intramuscularly as nucleoside-modified mRNA encapsulated in a lipid nanoparticle (LNP) delivery system. LNPs protect the mRNA from degradation by endonucleases during extracellular transit and carry it into cells. The LNP is a four-component mixture of neutral phospholipids, cholesterol, PEG-lipids, and an ionizable cationic lipid (ALC-0315), where the ionizable fraction drives mRNA complexation and later membrane interactions. Physical characterization shows that Comirnaty LNPs lack such intraliposomal pH gradient despite formulation at acidic pH, and cryo-TEM reveals a granular, solid core enclosed by mono- and bilipid layers rather than an aqueous-core vesicle. The encapsulated mRNA encodes the spike glycoprotein captured in its prefusion conformation. After injection, LNPs exhibit restricted biodistribution with preferential drainage toward immune-relevant tissue, targeting the draining axillary lymph nodes, concentrating antigen expression where adaptive priming begins.

Cellular uptake. At the cell surface, the ionizable lipid ALC-0315 acquires positive charge under relevant pH conditions, which facilitates the fusion of LNPs with the cell membrane during internalization. LNPs internalize via both clathrin-dependent and clathrin-independent endocytosis, including macropinocytosis. In vivo after vaccination, spike mRNA transcripts appear in draining lymph nodes at high levels on day 1 and decrease sharply by day 3, with reads primarily restricted to monocyte/macrophage and migratory dendritic cell clusters, indicating rapid transfer to antigen-presenting cells.

Endosomal escape. Delivery is rate-limited at the endosomal escape step. In endosomes, ionizable lipids become protonated in the acidic endosomal environment, which promotes lipid exchange with anionic endosomal membrane phospholipids and drives membrane fusion, enabling mRNA release into the cytosol. Escape probability is highest from early endocytic/recycling compartments — early endocytic/recycling compartments have the highest probability for mRNA escape — and total uptake does not predict delivery because LNP formulations vary considerably in endosomal distributions. Super-resolution microscopy has captured mRNA escape from endosomal recycling tubules, and gold-labeling studies indicate that only ~2% of particles are released to the cytoplasm, underscoring endosomal escape as the dominant delivery bottleneck.

mRNA translation and spike protein structure. Once in the cytosol, ribosomes translate the mRNA to produce the spike (S) protein, which is then degraded into fragments presented as antigens to macrophages and dendritic cells. The immunogen is prefusion-stabilized: BNT162b2 encodes spike captured in its prefusion conformation, and two amino acid mutations, K986P and V987P — both lysine/valine to proline substitutions at the HR1–CH junction — lock the trimer in its metastable prefusion architecture and preserve key neutralizing epitopes. The nucleoside substitution N1-methylpseudouridine (1mψ) is integral to sustained translation: 1mψ allows mRNA to evade innate immune detection, dampening TLR7 and MDA5-mediated degradation and thereby increasing translational efficiency and antigen yield.

Innate immune activation. BNT162b2 vaccination induces a type I interferon–dominated innate programme. Transcriptomics demonstrate post-boost upregulation of IFI6, IFIT3, and ISG15 on day 24, consistent with engagement of an antiviral interferon-stimulated gene signature. Upstream sensing proceeds via MDA5: the CD8+ T cell response induced by BNT162b2 depends on type I interferon–dependent MDA5 signaling, rather than TLR2/3/4/5/7, inflammasome activation, or necroptosis/pyroptosis pathways. In parallel, a target-independent innate pathway operates through the LNP carrier itself: the LNP component primarily activates monocytes, upregulating antigen presentation and costimulatory molecules on monocytes, cDC1, and plasmacytoid DCs independently of mRNA, providing a carrier-driven adjuvant effect.

Adaptive immune response and clinical efficacy. These innate signals drive coordinated germinal center (GC) reactions and effector lymphocyte differentiation. In draining lymph nodes, vaccination induces expansion of GC B cells, T follicular helper (Tfh) cells, and plasma cells, and prime–boost immunization in rhesus macaques elicited authentic SARS-CoV-2 neutralizing geometric mean titers 10–18 times that of convalescent serum. At the molecular level, vaccine-induced neutralizing antibodies prevent infection by blocking the RBD from binding the ACE2 receptor, inhibiting the initial viral entry event. Cellular immunity provides a parallel and early-acting protective axis: spike mRNA vaccines mediate protection from severe disease as early as ten days after prime vaccination, a window when neutralizing antibodies are barely detectable, in which CD8+ T cells are important effector cells that are expanded early and maintained stably after boost. BNT162b2 vaccination fully protected the lungs of immunized rhesus macaques from infectious SARS-CoV-2 challenge, and protected the lower respiratory tract from the presence of viral RNA with no evidence of disease enhancement, consistent with the combined humoral and cellular effector mechanisms translating to clinical prevention of severe COVID-19.