International Journal of Clinical Case Reports, 2025, Vol.15, No.5, 228-238 http://medscipublisher.com/index.php/ijccr 229 2 Immune Characteristics of Elderly Patients and COVID-19 Vaccine Response Mechanism 2.1 Immune senescence and physiological changes Immunosenescence refers to the gradual decline in the function of the immune system with age, which affects both innate immunity and acquired immune responses. In the elderly population, this change is mainly manifested as a reduction in the number and types of new T cells and B cells, a continuous accumulation of memory cells and senescent cells, and a decline in the function of cells responsible for recognizing pathogens. These changes can weaken the body's response to neoantigens, making people more susceptible to infection and leading to a decline in the effectiveness of vaccination (Grifoni et al., 2020). In addition, the elderly often experience a condition known as "chronic mild inflammation". This inflammation can further affect the normal functioning of the immune system and increase the possibility of developing severe illness after COVID-19 infection (Goyani et al., 2024). Physiological changes in the elderly, such as having comorbidities (like hypertension, diabetes, and chronic kidney disease), can also affect immune function and the response to vaccines. Immunosenescence combined with these comorbidities will increase the risk of severe illness and death among the elderly after they are infected with COVID-19. The immune senescence indicators observable in the laboratory include a decrease in the ratio of CD4/CD8 cells, a reduction in naive T cells, and an increase in the number of fully differentiated T cells and monoclonal T cells. These indicators all suggest that the recovery of the elderly after infection may be poorer, and the efficacy of vaccines may also be reduced (Cunha et al., 2020). 2.2 Mechanism of action of COVID-19 vaccines COVID-19 vaccines include mRNA vaccines, adenovirus vector vaccines, inactivated vaccines and subunit vaccines. Their aim is to stimulate humoral immunity (exerted by antibodies) and cellular immunity (exerted by T cells). These vaccines introduce the spike protein of the novel coronavirus (SARS-CoV-2) or its genetic material, causing the immune system to produce neutralizing antibodies. These antibodies prevent the virus from entering human cells by targeting the binding process of the spike protein to the ACE2 receptor (Şahin et al., 2020). In addition, vaccines can also promote the proliferation of CD4+T cells and CD8+T cells targeting the spike protein, which is important for clearing the virus and forming long-term immune memory (Grifoni et al., 2020). Effectively stimulating antibody and T-cell responses is the core to achieving an ideal protective effect, which is particularly important for the elderly whose immune functions are often weak. Booster shots can enhance the immune response levels of B cells and T cells and improve the decreased response caused by insufficient immunity in the elderly after the initial vaccination (Xu et al., 2023). Meanwhile, the use of adjuvants in vaccines and the continuous optimization of their composition can also help alleviate the weakened immune response caused by aging. 2.3 Differences in immune responses among the elderly population Compared with young people, the antibody and T-cell responses stimulated by vaccines in the elderly are relatively lower in both intensity and duration after vaccination. Studies have found that after receiving one or two doses of the COVID-19 vaccine, the neutralizing antibody levels in the elderly significantly decline, and their ability to block the ACE2 receptor also weakens. This may lead to a decrease in their protective ability against infection and virus variants such as Delta. In addition, a high number of chronic diseases and male gender are also associated with poor vaccine responses in the elderly (Brockman et al., 2021; Li, 2024). The COVID-19 vaccine is still effective in reducing the risk of severe illness and death among the elderly, although its protective effect is usually not as ideal as that among young people. The immune responses of different elderly people vary greatly. Elderly people with few underlying diseases or those who have received certain vaccines (such as adenovirus vector vaccines, mRNA booster vaccines) have a longer duration of immune memory retention and better vaccination effects (Xu et al., 2023). These results suggest that it is necessary to develop a specific vaccination plan for the elderly. According to the individual health conditions of the elderly, they can be given priority to receive booster shots, so as to provide them with more effective protection (Brockman et al., 2021).
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