International Journal of Clinical Case Reports, 2025, Vol.15, No.5, 228-238 http://medscipublisher.com/index.php/ijccr 230 3 Characteristics of Immune Response in Elderly Patients after COVID-19 Vaccination 3.1 Characteristics of humoral immune response After the elderly receive the COVID-19 vaccine, the intensity and duration of the humoral immune response are usually relatively weak. Compared with young people, after receiving one or two doses of mRNA vaccines, the antibody levels against the receptor binding domain (RBD) of the spike protein in the elderly significantly decline, the competitiveness of the ACE2 receptor decreases, and its ability to fight the virus also weakens. Moreover, over time, these immune responses fade away more rapidly, which may weaken the protective effect of antibodies, especially against virus variants such as Delta. The presence of multiple chronic diseases and male gender can make the humoral immune response of such people weaker (Brockman et al., 2021; Notarte et al., 2021). Getting a booster shot can significantly enhance the humoral immunity level of the elderly. Receiving a third dose of the vaccine can bring the antibody concentration and antiviral activity of the elderly to a level similar to that of young people. However, the response to variants such as Omicron is usually still relatively low. The most common reason for a weak humoral response after three doses of vaccination is having multiple chronic diseases, not just an issue of age. This indicates that it is very important to promote booster shots to this group of people (Figure 1) (Li et al., 2022; Mwimanzi et al., 2022; Muller et al., 2022). Figure 1 Anti-omicron IgG binding and ACE2 displacement activities 1 month after the second and third COVID-19 vaccine doses (Adopted from Mwimanzi et al., 2022)
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