Two groundbreaking preclinical studies published in Nature Communications and Scientific Reports reveal that intranasal COVID-19 vaccination strategies significantly outperform traditional intramuscular approaches in generating mucosal immunity and providing long-term protection against SARS-CoV-2 infection.
Albumin-Fusion Technology Enables Effective Mucosal Vaccination
The first study, published in Nature Communications, introduces an innovative vaccine design that fuses viral antigens to albumin, targeting the neonatal Fc receptor (FcRn) for enhanced mucosal delivery. Researchers demonstrated that fusing the SARS-CoV-2 receptor-binding domain (RBD) to mouse serum albumin (MSA) or engineered human serum albumin variants induced robust antibody responses when administered intranasally.
In BALB/c mice, intranasal vaccination with RBD-MSA generated strong RBD-specific IgG responses in both serum and bronchoalveolar lavage fluid (BALF), while unfused RBD produced barely detectable levels. Importantly, only the albumin-fused vaccine induced RBD-specific IgA antibodies in BALF, which are crucial for mucosal immunity.
The study validated this approach across four different mouse strains, including human FcRn transgenic mice, to address species differences in albumin-receptor interactions. When compared to intramuscular mRNA vaccination (BioNTech-Pfizer), intranasal RBD-MSA vaccination produced comparable systemic IgG responses while uniquely generating mucosal IgA responses throughout the respiratory tract.
Challenge studies in K18-hACE2 transgenic mice demonstrated complete protection against SARS-CoV-2 infection. Mice vaccinated with RBD-MSA showed no weight loss, significantly lower viral loads in lungs, and reduced inflammation compared to controls or mice receiving unfused RBD.
The researchers extended their findings to influenza vaccination, showing that hemagglutinin fused to albumin provided 86% survival against lethal influenza challenge, compared to 43% for transferrin-fused controls. Long-term memory studies revealed complete protection even 4.5 months after initial vaccination.
Intranasal ChAdOx1 Vaccination Provides Durable Upper Respiratory Tract Protection
The second study, published in Scientific Reports, compared intranasal ChAdOx1 nCoV-19 vaccination to intramuscular mRNA vaccination in a prime-boost strategy. C57BL/6J mice received two doses of mRNA vaccine intramuscularly, followed by either a third intramuscular mRNA dose or an intranasal ChAdOx1 nCoV-19 booster.
The intranasal group demonstrated significantly higher IgA titers in nasal-associated lymphoid tissue (NALT) and BALF at day 14, with responses remaining elevated at day 84 for most variants. In contrast, intramuscular vaccination alone failed to induce meaningful mucosal IgA responses.
Cellular immunity analysis revealed that intranasal vaccination uniquely induced spike-specific tissue-resident memory (TRM) T cells in the upper respiratory tract. Up to 21.5% of spike-specific CD8+ T cells in nasal turbinates expressed the TRM markers CD103 and CD69, with the highest levels observed at day 84. The intranasal group also showed increased frequencies of antigen-presenting cells and resident B cells in lung tissue.
Immunohistochemical analysis of NALT revealed significant structural changes following intranasal vaccination, including increased size, enhanced T and B cell infiltration, and formation of germinal center-like structures. Cell proliferation markers peaked at day 14, indicating robust immune activation.
Superior Protection Against Viral Challenge
Challenge studies using K18-hACE2 transgenic mice and SARS-CoV-2 Omicron EG.5.1 variant demonstrated the superior protective efficacy of intranasal vaccination. At 14 days post-vaccination, both vaccine groups provided equivalent protection. However, at 84 days post-vaccination, only the intranasal group maintained complete protection in the upper respiratory tract.
Viral RNA was undetectable in nasal turbinates of intranasally vaccinated mice at day 2 post-challenge, while intramuscularly vaccinated mice showed viral loads comparable to unvaccinated controls. Both groups maintained protection in the lower respiratory tract.
Transcriptomic analysis revealed enhanced adaptive immune responses in nasal turbinates of intranasally vaccinated animals, including upregulation of B and T cell receptor signaling pathways and increased expression of genes related to antibody secretion. The peroxisome proliferator-activated receptor (PPAR) signaling pathway, associated with B cell differentiation and antibody production, was significantly enriched in the intranasal group.
Clinical Implications and Future Directions
These studies provide compelling evidence that mucosal vaccination strategies could address current limitations of intramuscular COVID-19 vaccines, particularly in preventing upper respiratory tract infections and potentially reducing transmission. The albumin-fusion technology offers a platform for developing needle-free vaccines with enhanced patient compliance.
The durability of mucosal immunity observed in these studies is particularly noteworthy, as IgA responses and TRM cells remained elevated for extended periods in the upper respiratory tract. This contrasts with the more transient responses typically observed in lung tissue.
Both studies demonstrated cross-reactivity against multiple SARS-CoV-2 variants, though responses were stronger against earlier variants compared to Omicron sublineages. The ability to induce both systemic and mucosal immunity positions these approaches as promising next-generation vaccine strategies.
The research highlights critical anatomical differences between upper and lower respiratory tract immunity, with the upper respiratory tract showing more durable immune responses. This finding has important implications for vaccine design, as the upper respiratory tract serves as the primary site of SARS-CoV-2 infection and transmission.
While these preclinical results are highly encouraging, the researchers acknowledge the need for human clinical trials to validate these findings and optimize delivery methods for the complex human respiratory mucosa. The practical advantages of intranasal administration, including reduced invasiveness and potential for improved vaccine uptake, make this approach particularly attractive for global vaccination efforts.
