Journal of Vaccine Research 2024, Vol.14, No.5, 231-242 http://medscipublisher.com/index.php/jvr 234 However, booster doses have been shown to significantly improve protection against Omicron. A Danish study reported that the third dose increased VE against Omicron infection to 57.7% shortly after the booster, although this protection also waned over time (Gram et al., 2022). Similarly, a meta-analysis found that a booster dose restored protection against Omicron infection up to 51.1% within three months, although this waned to 32.8% within six months (Mohammed et al., 2023). Despite the reduced effectiveness against infection, vaccines have maintained high effectiveness against severe outcomes. For instance, the VE against Omicron-associated hospitalization remained high, with estimates of 95.5% or above shortly after vaccination (Gram et al., 2022). While the primary vaccination series provides limited and short-lived protection against Omicron infection, booster doses significantly enhance and prolong protection. However, the effectiveness of these boosters also wanes over time, indicating the potential need for regular booster doses to maintain high levels of immunity against the Omicron variant. 4 The Waning of Vaccine Protection and the Efficacy of Booster Shots 4.1 Waning vaccine efficacy The efficacy of COVID-19 vaccines has been shown to decline over time, a phenomenon observed across various vaccine types and populations. Studies have consistently reported a significant reduction in antibody levels and vaccine effectiveness within months following the completion of the primary vaccination series. For instance, a systematic review highlighted that the peak humoral response is typically reached 21-28 days after the second dose of mRNA vaccines, after which antibody levels progressively diminish. By 4-6 months post-vaccination, there is a notable decline in both anti-receptor binding domain immunoglobulin G (IgG) and anti-spike IgG levels, with reductions ranging from 55% to 95% depending on the time frame and specific antibody measured (Notarte et al., 2021). This waning immunity is not limited to mRNA vaccines. Similar trends have been observed with other vaccine platforms, including inactivated vaccines. For example, a study on the BBIBP-CorV vaccine reported a significant decrease in antibody levels three months after the second dose, underscoring the need for booster doses to maintain adequate protection (Chansaenroj et al., 2022). The decline in vaccine efficacy is particularly concerning in the context of emerging variants of concern (VOCs) such as Delta and Omicron, which have shown the ability to partially evade immunity conferred by the initial vaccination series (Menegale et al., 2023). 4.2 Immune response mechanism of boosters Booster doses are designed to reinvigorate the immune system by enhancing both humoral and cellular responses. The administration of a booster dose has been shown to significantly increase neutralizing antibody titers against SARS-CoV-2, including its variants. For instance, a meta-analysis demonstrated that the first booster dose of COVID-19 vaccines could induce higher seroconversion rates and neutralizing antibody titers compared to the full vaccination series, thereby providing robust cellular immune responses (Xu et al., 2023). The mechanism behind this enhanced response involves the reactivation of memory B cells and T cells, which are crucial for long-term immunity. Boosters help in generating a more potent and durable immune response by increasing the quantity and quality of antibodies. This is particularly important for older adults, who may have a weaker initial response to vaccination. A study comparing homologous and heterologous mRNA vaccine boosters found that heterologous boosters (e.g., BNT162b2 followed by mRNA-1273) induced a stronger neutralizing response against the Omicron variant, especially in older individuals (Poh et al., 2022). 4.3 Clinical effects of boosters The clinical benefits of booster doses are evident in their ability to reduce the risk of SARS-CoV-2 infection and severe COVID-19 outcomes. A comprehensive meta-analysis revealed that individuals who received a booster dose had significantly lower risks of infection, ICU admission, and death compared to those who only completed the primary vaccination series. The risk ratios for these outcomes were markedly lower in the booster group, indicating the substantial protective effect of booster doses (Xu et al., 2023).
RkJQdWJsaXNoZXIy MjQ4ODYzNQ==