Background

From an immunological perspective, simple aging due to the passage of time and frailty—a decline in biological vitality—exhibit entirely different characteristics. Frailty refers to a clinical state of vulnerability where the ability to maintain biological homeostasis drops sharply, increasing susceptibility to stress. The elderly in a state of frailty are not only more prone to various infectious diseases but also exhibit significantly lower expected protective effects when vaccinated.

Previously, the medical community has primarily divided age groups based on chronological age—the physical years passed since birth—when establishing vaccination guidelines. However, even at the same age of 75, it is difficult to evaluate the immune functions of an elderly person maintaining an active daily life on the same scale as one weakened by chronic diseases or sarcopenia. This is because 'inflammaging' (chronic low-grade inflammatory response), abnormal hematopoiesis, and the simultaneous dysfunction of innate and adaptive immunity are complexly intertwined to distort vaccine responses.

During the COVID-19 pandemic, Lipid Nanoparticle (LNP)-based messenger RNA (mRNA) vaccines were widely distributed globally. Due to their rapid development speed and powerful immune induction, they became a mainstay in responding to infectious disease crises. However, in actual vaccination settings, limitations such as lower antibody formation rates or shorter duration of immunity in vulnerable elderly populations compared to healthy adults have been repeatedly reported. This was due to applying uniform doses and formulations without precisely considering frailty indicators, which are key metrics for explaining diversity within the elderly population.

Key Findings

The research team systematically analyzed the mechanistic pathways through which frailty-associated immunosenescence affects the immunogenicity, clinical efficacy, and in vivo safety of LNP-mRNA vaccines. In frail individuals, myeloid bias in bone marrow hematopoiesis is intensified, reducing lymphocyte production, and circulating innate immune cells are in a refractory state due to continuous exposure to chronic inflammatory signals. Consequently, it was revealed that the migratory ability and antigen-presenting functions of dendritic cells—which recognize vaccine antigens and move to lymph nodes to deliver them to T cells—are significantly inhibited.

Clear disparities were also observed in clinical data analysis. A follow-up of a cohort vaccinated with COVID-19 mRNA vaccines revealed that, compared to the non-frail elderly group, the frail group exhibited lower post-vaccination neutralizing antibody titers and a more rapid decline in antibody persistence. T-cell memory immune formation rates were also reduced, exhibiting a pattern of weakened defense against reinfection. However, it was confirmed that heterogeneity exists between results due to differences in frailty assessment tools, and high-quality data specifically tracking only the frail patient group is still lacking.

Researchers noted the inherent flexibility of the mRNA platform. By leveraging the advantage of flexible modification of mRNA sequences and LNP compositions, precision vaccine design can compensate for deficiencies in immunosenescence. Specifically, alternatives were proposed, such as adjusting the lipid composition of LNPs to increase in vivo half-life or attaching ligands targeting dendritic cells, as well as optimizing administration routes and doses to increase delivery efficiency to the lymph nodes of the elderly. Additionally, design strategies such as co-loading vaccine cargo with cytokine transcripts that serve an adjuvant role alongside antigen sequences, or adopting multivalent vaccine approaches, were identified as effective solutions.

Significance and Outlook

This review provides a compass for shifting vaccine strategies in an aging society from being centered on chronological age to being based on frailty indicators, which focus on biological functional assessment. If individual frailty indicators are included as evaluation endpoints from the clinical design stage of vaccine candidates, insufficient immune responses in vulnerable populations can be identified early and formulations adjusted. This serves as evidence that LNP vaccine technology can evolve beyond a platform that simply provides rapid prevention of infectious diseases into a customized therapeutic platform for the immunocompromised.

Along with the engineering improvements of the vaccine itself, host-directed therapeutic strategies that improve the patient's biological environment have emerged as a key task. A robust immune response can be expected when the management of underlying chronic diseases, correction of protein and micronutrient deficiencies, and treatment to alleviate chronic inflammation in frail elderly individuals are conducted alongside vaccination programs. There is an urgent need to establish a comprehensive prevention model that integrates pharmacological approaches and clinical geriatric interventions.

Future tasks include the standardization of frailty assessment criteria and the establishment of large-scale prospective clinical cohorts. Because the frailty assessment scales used by research institutions worldwide differ, direct comparison of vaccine efficacy data is currently difficult. The establishment of standardized frailty biomarkers must be followed by the determination of optimal dosing regimens and vaccination intervals tailored to frail patients through prospective comparative clinical trials.