Seasonal influenza is a significant global health concern, causing millions of severe cases and hundreds of thousands of deaths annually. Vaccination remains the most effective preventative measure, but current vaccine effectiveness varies (10-60%). Improving vaccine efficacy requires a deeper understanding of the immune mechanisms involved and exploring alternative delivery methods. Higher antigen doses, adjuvants, and alternative routes of administration have all been proposed as strategies for improvement. Targeting the skin, rich in dendritic cells and lymphatic networks, offers potential advantages over intramuscular injection for stimulating B and T cell responses. Microneedle patches (MNPs) provide a promising skin delivery system, offering several potential advantages over traditional injections, such as ease of self-administration, simplified distribution and storage, cost-effectiveness (reduced administration costs, cold chain requirements, and sharps waste), and improved safety (eliminating needle-stick injuries). Previous research in animal models and a preliminary human trial demonstrated the safety and immunogenicity of MNP-delivered influenza vaccines. This study aimed to conduct a broader immunological analysis of a subset of participants from a phase 1 clinical trial to compare MNP and IM delivery methods.

The introduction extensively reviews the literature on influenza vaccine effectiveness, limitations of current delivery methods, and the potential benefits of skin-based immunization. It highlights the advantages of microneedle patches in terms of ease of administration, cost-effectiveness, and safety compared to intramuscular injections. Existing literature supporting the use of intradermal vaccination and other skin delivery modalities is also reviewed, emphasizing the immune response advantages of skin-based delivery, in contrast to conventional methods.

A partially blinded, randomized, placebo-controlled, phase 1 clinical trial was conducted involving healthy adults aged 18–49 years who had not received the influenza vaccine during the 2014–2015 season. Participants were randomly assigned to receive the 2014–2015 seasonal trivalent influenza vaccine either via MNP or IM injection. Detailed immunologic analyses were performed on a subset of participants (n=11 per group). Blood samples were collected at various time points (Days 0, 2–3, 10–26, 30, and 166–194 post-vaccination). Serological responses were assessed using hemagglutination inhibition (HAI) and neuraminidase inhibition (NAI) assays. Cellular immune responses were evaluated using intracellular cytokine staining (ICS), measuring cytokines such as IL-2, IL-21, IFN-γ, and CD154 (CD40L) in CD4+ T cells. Circulating T follicular helper (cTfh) cells were also quantified. Antibody binding affinity was determined using surface plasmon resonance (SPR). Influenza virus-specific IgG-secreting memory B cells (MBCs) were measured using ELISpot assays. Cytokine and chemokine levels were assessed using Luminex assays. Statistical analyses included Student’s t-tests, chi-square tests, Mann–Whitney Wilcoxon tests, and Bonferroni correction for multiple comparisons.

HAI responses were similar between the MNP and IM groups at 1 and 6 months post-vaccination. However, MNP vaccination resulted in significantly higher NAI titers for all three influenza strains at both Day 28 and Day 180. MNP vaccination also induced a significantly higher percentage of circulating T follicular helper cells (cTfh) at Day 8 post-vaccination. The MNP group showed an early increase in pro-inflammatory cytokines (IP-10, IL-1β, and IL-8), while also exhibiting higher levels of IL-5 and IL-13 (allergenic inflammation). There were no significant differences in antibody avidity or influenza-specific IgG-secreting memory B cell responses between groups. While there was a numerical increase in IL-2+CD40L+CD4+ T cells in the MNP group, this difference was not statistically significant.

The findings demonstrate that MNP-delivered influenza vaccine is not only safe and well-tolerated, as shown in previous studies, but also induces a robust immune response comparable to or exceeding that of IM injection. The superior NAI response observed with MNP delivery is particularly noteworthy, as NAI antibodies are considered an independent correlate of protection. The increased cTfh cell response suggests a potential enhancement of B cell help and antibody production. The early pro-inflammatory response could contribute to the faster onset of immune response, while the increased allergenic inflammation might account for reported local pruritus. This study provides strong evidence supporting the advantages of MNP delivery for influenza vaccination.

Microneedle patch delivery of inactivated influenza vaccine offers significant logistical advantages and promising immunological benefits. This study demonstrated superior neuraminidase inhibition titers and enhanced T follicular helper cell responses in the MNP group compared to the IM group. Future research should focus on larger-scale clinical trials to confirm these findings and investigate the long-term efficacy and safety of MNP-delivered influenza vaccines, particularly in diverse populations, including those who are more vulnerable to seasonal influenza.

The small sample size (n=11 per group) is a limitation of this phase 1 study. The potential influence of prior influenza vaccination in some participants could have impacted the observed immune responses. The lack of assessment of CD8+ T cell responses is another limitation. The study did not assess immunological responses in participants who received a placebo MNP, making it difficult to isolate the effects of the MNP itself versus the antigen delivery.