Journal of Vaccine Research 2024, Vol.14, No.5, 231-242 http://medscipublisher.com/index.php/jvr 236 Another study highlighted that the effectiveness of two doses of the BNT162b2 vaccine was 93.7% against the Alpha variant and 88.0% against the Delta variant. For the ChAdOx1 nCoV-19 (AstraZeneca) vaccine, the effectiveness was 74.5% against the Alpha variant and 67.0% against the Delta variant (Bernal et al., 2021). These findings underscore the importance of full vaccination, as partial vaccination was significantly less effective, with VE of 59.0% against Alpha and 52.6% against Delta (Zeng et al., 2021). 5.2 Case study 2: vaccine efficacy in South Africa against the beta variant In South Africa, the Beta (B.1.351) variant posed significant challenges due to its ability to partially evade immune responses. The efficacy of various vaccines against this variant has been variable. For instance, the ChAdOx1 nCoV-19 (AstraZeneca) vaccine showed an efficacy of only 10.4% in preventing mild to moderate COVID-19 caused by the Beta variant (Fiolet et al., 2021). In contrast, the NVX-CoV2373 (Novavax) vaccine demonstrated an efficacy of 50% in South Africa, where the Beta variant was dominant (Fiolet et al., 2021). A systematic review and meta-analysis reported that full vaccination with mRNA vaccines provided moderate effectiveness against the Beta variant, with a VE of 70.7% (Zeng et al., 2021). This was corroborated by seroneutralization studies, which showed a significant reduction in neutralizing activity against the Beta variant for mRNA vaccines, Sputnik V, and CoronaVac (Fiolet et al., 2021). Despite these challenges, full immunization with mRNA vaccines was effective in preventing severe outcomes, including hospitalization and death, even against the Beta variant (Fiolet et al., 2021). 5.3 Case study 3: vaccine effectiveness in denmark against the Omicron variant Denmark has been at the forefront of monitoring the effectiveness of COVID-19 vaccines against the Omicron (B.1.1.529) variant. A nationwide cohort study in Denmark revealed that two doses of mRNA vaccines provided limited and short-lived protection against Omicron infection, with a VE of only 39.9% shortly after vaccination, which waned to 4.4% over time (Gram et al., 2022). However, the effectiveness against COVID-19 hospitalization remained high, with VE estimates of 95.5% or above for Omicron after two or three doses (Gram et al., 2022). The study also highlighted the importance of booster doses. The third dose of mRNA vaccines significantly increased protection against Omicron infection, with a VE of 57.7% shortly after the booster dose (Gram et al., 2022). This aligns with findings from other studies, which reported that mRNA vaccine boosters reestablished effectiveness against Omicron, although to a lower extent compared to Delta and Alpha variants (Paul et al., 2023). 6 Comparison of Different Vaccine Types 6.1 mRNA vaccines: the performance of BNT162b2 and moderna against different variants mRNA vaccines, particularly BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna), have shown varying levels of effectiveness against different SARS-CoV-2 variants. During the Delta variant predominance, the effectiveness of two doses of mRNA vaccines against COVID-19-associated emergency department (ED) and urgent care (UC) encounters was 86% within 14-179 days post-vaccination, dropping to 76% after 180 days. However, a third dose significantly boosted effectiveness to 94% (Thompson et al., 2022). Against the Omicron variant, the effectiveness of two doses dropped substantially to 52% within 14-179 days and 38% after 180 days, but a third dose increased effectiveness to 82% (Thompson et al., 2022). This pattern was also observed in hospitalizations, where the effectiveness of two doses against Delta was 90% within 14-179 days and 81% after 180 days, while a third dose increased it to 94%. For Omicron, the effectiveness was 81% within 14-179 days and 57% after 180 days, with a third dose boosting it to 90% (Thompson et al., 2022). A systematic review and meta-analysis further confirmed that booster doses of mRNA vaccines provide better protection against Omicron infections compared to the full dose. The booster dose showed a 22% increase in protection against any Omicron infection, 20% against severe infections, and 22% against symptomatic infections within three months (Pratama et al., 2022). However, the effectiveness of the booster dose also waned over time, indicating the need for regular booster shots to maintain high levels of protection (Pratama et al., 2022).
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