International Journal of Clinical Case Reports 2024, Vol.14, No.3, 117-131 http://medscipublisher.com/index.php/ijccr 120 However, some variations in efficacy have been observed in specific populations. For example, a study on the effectiveness of a third dose of mRNA vaccines found that vaccine efficacy against COVID-19-associated emergency department and urgent care encounters was lower during the Omicron variant predominance compared to the Delta variant period (Thompson et al., 2022). Despite these variations, the overall efficacy of mRNA vaccines remains high, particularly with the administration of booster doses, which significantly enhance protection (Ioannou et al., 2022; Thompson et al., 2022). 3.3 Long-term efficacy and booster doses The long-term efficacy of mRNA vaccines has been a subject of ongoing research, particularly in the context of emerging variants and waning immunity. Studies have shown that vaccine-induced immunity can wane over time, necessitating booster doses to maintain high levels of protection. For instance, the effectiveness of mRNA vaccines against COVID-19-associated hospitalizations decreased over time but was significantly restored with a third (booster) dose (Ioannou et al., 2022; Thompson et al., 2022). Booster doses have proven to be highly effective in enhancing immunity. A study on the effectiveness of a third dose of mRNA vaccines found that the booster dose increased vaccine efficacy against SARS-CoV-2 infection, hospitalization, and death, particularly during the Omicron variant era (Ioannou et al., 2022). Another study reported that a bivalent mRNA booster dose provided strong protection against COVID-19-associated hospitalization in older adults, further emphasizing the importance of booster doses in maintaining long-term efficacy (Surie et al., 2022). 4 Safety Profile of mRNA Vaccines 4.1 Short-term safety The short-term safety profile of mRNA vaccines has been extensively studied across various clinical trials. In a phase 1 trial evaluating mRNA vaccines against H10N8 and H7N9 influenza viruses, the vaccines demonstrated favorable safety and reactogenicity profiles. No vaccine-related serious adverse events were reported, and the most common adverse reactions were mild to moderate, including pain at the injection site, headache, and fatigue (Feldman et al., 2019). Similarly, the mRNA-1273 SARS-CoV-2 vaccine showed an acceptable safety profile in a phase 2 trial, with the most common adverse reactions being pain at the injection site, headache, and fatigue. Only one serious adverse event, deemed unrelated to the vaccine, was reported (Chu et al., 2021). Further supporting these findings, a phase 3 trial of the mRNA-1273 vaccine reported that moderate, transient reactogenicity occurred more frequently in the vaccine group compared to the placebo group. Serious adverse events were rare and occurred at similar rates in both groups (Baden et al., 2020). Another study on the BNT162b2 mRNA vaccine also reported a favorable safety profile, with the most common adverse events being mild to moderate pain at the injection site, fatigue, and headache (Thomas et al., 2021). These findings indicate that mRNA vaccines are generally well-tolerated in the short term, with most adverse events being mild and transient. 4.2 Long-term safety Long-term safety data for mRNA vaccines are still being collected, but initial findings are promising. In a phase 3 trial of the mRNA-1273 vaccine, the median follow-up was 5.3 months, and no safety concerns were identified. The vaccine continued to be efficacious in preventing COVID-19 illness and severe disease, with an acceptable safety profile (Sahly et al., 2021). Similarly, the BNT162b2 vaccine showed a favorable safety profile through six months of follow-up, despite a gradual decline in vaccine efficacy. Serious adverse events were rare, and the incidence was similar between the vaccine and placebo groups (Thomas et al., 2021). A systematic study and meta-analysis of COVID-19 mRNA vaccines in children aged 5 to 11 years also provided insights into long-term safety. The study found that while most children developed local adverse events, severe adverse events were rare and resolved within several days (Watanabe et al., 2023). Another study on the safety and immunogenicity of a mRNA rabies vaccine reported that the vaccine was generally safe with a reasonable
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