Improving Intradermal Vaccination in Mice with a SARS-CoV-2 mRNA Vaccine

Intradermal (ID) vaccination offers immunological advantages over conventional intramuscular (IM) delivery by engaging the skin's rich population of antigen-presenting cells, but it remains technically challenging in preclinical models. In a recent study, Onishi and colleagues set out to address this barrier by developing a standardized hollow microneedle (MN) platform with enhanced delivery consistency. Using SARS-CoV-2 mRNA vaccination in mice, they demonstrated that skin-thickness-guided applicator design dramatically improves ID injection reproducibility and enables rigorous immunogenicity assessment.

Developing the Platform

The investigators first characterized dorsal skin thickness in BALB/c mice from 7 to 13 weeks of age. Caliper-measured skinfold thickness varied considerably over time, ranging from a mean of 354.2 µm at 7 weeks to 585.3 µm at 13 weeks. By comparison, measurements of the actual skin layer using cryosections—thin, frozen tissue sections examined microscopically—showed much smaller age-related differences. This suggested that changes in the effective skinfold, rather than the dermal layer alone, were important for successful dosing.

To account for this variability, the researchers developed a pinch-type applicator with interchangeable rings that stabilized the folded skin under controlled tension. A single hollow microneedle delivered 10 µL into the skin, while supporting needles and depth-control pillars helped standardize penetration. Before each administration, skinfold thickness was measured and the appropriate ring size selected for that animal. Successful intradermal delivery was defined by formation of a wheal at least 2 × 2 mm without leakage.

Across the method-development experiments, skin-thickness-guided control increased the injection-attempt success rate from 5.0% (16/320 injections) without pinching diameter control to 51.5% (100/194 injections). In a subsequent feasibility assessment, a predefined redosing procedure ultimately achieved successful intradermal delivery for all 21 intended administration events.

Putting the Platform to the Test

The investigators next conducted an exploratory vaccination study in male C57BL/6J mice. Animals were weight-stratified and randomized into five groups of five, although one control animal died for reasons unrelated to study procedures. Mice received either an intramuscular control injected with firefly luciferase mRNA, 0.5 µg of SARS-CoV-2 spike mRNA–lipid nanoparticles intramuscularly, or 0.1, 0.5, or 2.5 µg intradermally. Vaccinations were administered twice, 2 weeks apart.

Two weeks after the booster, intradermal vaccination produced detectable anti-receptor-binding domain (RBD) IgG responses. At the matched 0.5-µg dose, the intradermal group demonstrated a statistically significant increase versus the negative control (p < 0.05), whereas the intramuscular group did not meet statistical significance under these fixed-dilution assay conditions. Importantly, the authors caution that this finding does not establish superior humoral immunity with intradermal administration as the comparison was limited to a single serum dilution rather than full dose-response characterization.

Flow cytometry revealed more nuanced cellular responses. At matched doses, intradermal administration tended to produce higher proportions of IFN-γ– and TNF-α–positive CD8+ T cells than intramuscular administration, but these differences were not statistically significant. However, the intradermal 2.5 µg higher dose demonstrated significant increases in IFN-γ–positive CD8+ T cells, TNF-α–positive CD8+ T cells, and a marked increases in IFN-γ– and TNF-α–positive CD4+ T cells. Notably, these cellular immune responses continued to increase with higher intradermal dosing even though fixed-dilution antibody responses showed only modest additional increases.

A Platform, Not Yet Evidence of Route Superiority

The findings are best viewed as proof of concept for the delivery method rather than evidence that intradermal vaccination is superior to intramuscular vaccination. The small exploratory study lacked a fully dose-matched intramuscular series, evaluated vaccine responses only in male mice, and measured antibodies using a single fixed serum dilution rather than endpoint titers or neutralizing antibody activity. The authors also note that their platform's performance in older animals or immunodeficient mouse strains remains unexplored.

Still, improving dosing consistency may make it easier to incorporate intradermal administration into early small-animal studies and more rigorously investigate how dose and delivery route shape humoral and cellular immunity.

Reference:

Onishi S, Suzuki Y, Sugimoto K, et al. Improving intradermal dosing consistency in mice using a hollow microneedle: A model study with a SARS-CoV-2 mRNA vaccine. Vaccine. Published online September 5, 2026. doi:10.1016/j.vaccine.2026.129111