International Journal of Clinical Case Reports, 2025, Vol.15, No.5, 228-238 http://medscipublisher.com/index.php/ijccr 231 3.2 Cellular immune responses and protective effects After the elderly receive the COVID-19 vaccine, their immune response at the cellular level declines. Compared with young people, the peak levels of various cytokines (such as IFN-γ, TNF, IL-2, and IL-6) in the elderly (especially those who have been living in elderly care institutions for a long time) are significantly lower after the first vaccination. Although booster shots can increase the content of some cytokines, the responses of the elderly to IFN-γ and TNF are usually still weak, indicating that cellular immunity has not been fully restored (Hayati et al., 2024). Furthermore, the breadth and intensity of T-cell responses (including the range of viral types that T-cell receptors can recognize) also weaken with age, which may weaken the body's ability to control viral replication and respond to new variants (Delmonte et al., 2023). Despite some shortcomings, getting vaccinated against COVID-19 can still provide important protection for the elderly and significantly reduce the risk of severe illness and death. Even if the overall antibody immunity and cellular immunity levels decline, as long as there is still a certain response, it will help reduce hospitalizations and deaths. It is worth noting that elderly people who have been infected with the novel coronavirus or have mixed immunity (infection plus vaccination) usually develop stronger antibody and T-cell responses, and thus achieve better protective effects (Chen et al., 2020; Li et al., 2022). 3.3 Factors affecting immune response A variety of factors can affect the immune response of elderly patients after vaccination against COVID-19. Age itself is an independent factor leading to weakened humoral and cellular immunity, and the types and quantities of chronic diseases (such as hypertension, diabetes and kidney disease) will further weaken the immune effect stimulated by vaccines. Furthermore, male gender and the use of certain immunosuppressive drugs may also lead to a reduced level of immune response (Brockman et al., 2021; De Boer et al., 2022). The type of vaccine and the vaccination plan have a significant impact on the immune response effect of the elderly. mRNA vaccines usually cause stronger reactions than inactivated vaccines or vector vaccines. Additional booster shots are particularly helpful in overcoming age-related immune deficiencies (Li et al., 2022; Muller et al., 2022). The timing of vaccination, previous infection with COVID-19, and overall health status (including fattening and nutritional status) will further affect the strength and duration of humoral and cellular immunity in this group of people (Notarte et al., 2021). 4 Adverse Events and Influencing Factors After Vaccination Against the Novel Coronavirus 4.1 Common mild adverse reactions Most of the adverse reactions that occur in the elderly after receiving the COVID-19 vaccine are relatively mild. Common manifestations include local symptoms such as pain, redness and swelling at the vaccination site, as well as systemic reactions such as fever, fatigue, headache, muscle aches, chills and dizziness. Most of these symptoms do not require medication and usually improve on their own within a few days (Figure 2) (Aboud et al., 2022). The probability of these mild reactions in the elderly is similar to or slightly lower than that in young people. Among them, local symptoms are more common than systemic symptoms (Lokendra et al., 2021; Wang et al., 2022). The frequency of mild reactions caused by different types of vaccines varies. Compared with inactivated vaccines and viral vector vaccines, mRNA vaccines are usually more likely to cause local and systemic reactions (Li et al., 2022). It should be noted that most of these reactions do not require hospitalization or special treatment and should not affect the continued vaccination of the elderly, because the protective effect brought by vaccination is far greater than the possible risks (Xu et al., 2023). 4.2 Serious adverse events and their occurrence mechanisms It is particularly rare for the elderly to experience severe adverse reactions after receiving the COVID-19 vaccine. Serious problems that may occur include cardiovascular aspects (such as myocarditis, pericarditis, arrhythmia, transient ischemic attack), thrombotic events, severe allergic reactions and neurological complications. This kind of situation is generally very rare. Statistical results show that for every one million doses of vaccine administered,
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