International Journal of Clinical Case Reports, 2025, Vol.15, No.4, 190-199 http://medscipublisher.com/index.php/ijccr 193 Recombinant vaccine RIV4 is also more effective than common IIV4 and can reduce the risk of flu-like symptoms in the elderly by 30%. Although all vaccines have certain protective effects, enhanced and recombinant vaccines are more suitable for the elderly living in nursing homes because they have better immune effects and stronger protective effects (Diazgranados et al., 2014; Dunkle et al., 2017; Lee et al., 2018; Ng et al., 2018; Lee et al., 2020; Gartner et al., 2022; Veroniki et al., 2024). 4 The Changing Trend of Immune Response 4.1 Common reactions after vaccination The elderly usually experience mild and transient discomfort after vaccination, such as pain, redness or swelling at the injection site, and sometimes low fever or physical discomfort (Bonduelle et al., 2025). The frequency and severity of these reactions are roughly the same as those of young people. However, some studies have pointed out that the elderly may have less systemic reactions due to weakened immune responses than the young (Govaert et al., 1994; Brydak et al., 2003). When using high-dose and other enhanced vaccines, there may be slightly more obvious local or systemic reactions, but most reactions are still within the safe range and do not cause serious problems. Influenza vaccine is generally well tolerated among the elderly population in elderly care institutions and is therefore still suitable for promotion and use (Govaert et al., 1994; Bonduelle et al., 2025). 4.2 Changes in antibody levels over time After vaccination, the antibody level of the elderly will increase significantly and reach the peak about 3 to 4 weeks after vaccination. However, the level of this antibody will gradually decline over time. After approximately 5~6 months, both the protective efficacy and quantity of the antibody will decrease. Therefore, vaccination is required every year to maintain the protective efficacy (Levine et al., 1987; Brydak et al., 2003; Kitamura et al., 2020; Sugishita et al., 2020). Regular annual vaccination can help maintain basal antibody levels, but the immune effect may weaken between different years, especially if the same vaccine strain is administered for several consecutive years. Nevertheless, the elderly still need to receive the influenza vaccine annually to maintain antibody levels in their bodies (Kitamura et al., 2020; Sugishita et al., 2020). 4.3 Characteristics of adverse reactions After influenza vaccination, the elderly generally only experience mild adverse reactions, and these reactions usually resolve spontaneously without the need for special treatment. Severe adverse events are very rare (Bonduelle et al., 2025). The most common is discomfort at the injection site, while systemic reactions such as fever or fatigue are less common and less severe in the elderly (Govaert et al., 1994; Brydak et al., 2003). High-dose or adjuvant vaccines may slightly increase the probability of local or mild systemic reactions, but do not significantly increase the risk of serious adverse events (Li, 2024). However, in concentrated environments such as elderly care institutions, it is still necessary to closely observe the reactions after vaccination in order to detect and handle any abnormal situations in a timely manner (Govaert et al., 1994; Bonduelle et al., 2025). 5 Factors Influencing Immune Responses 5.1 Individual physiological differences The immune response of the elderly to the influenza vaccine varies greatly, mainly influenced by age, genetic factors and previous infection experiences (Bernstein et al., 1999; Castrucci, 2018). The older the age, the more obvious the aging of the immune system, the decline of B cell function, the reduction of antibody production, resulting in poorer vaccine efficacy (Goodwin et al., 2006; Henry et al., 2019; Frasca et al., 2020; Frasca and Blomberg, 2020). Some elderly people still have a good immune response, but in others, antibodies do not increase significantly or decrease rapidly, which shows that there are significant differences in each person's physical condition (Bernstein et al., 1999; Song et al., 2010).
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