Journal of Vaccine Research 2024, Vol.14, No.5, 231-242 http://medscipublisher.com/index.php/jvr 233 3 Vaccine Effectiveness Against Variants 3.1 Protection against Alpha variant The Alpha variant (B.1.1.7) was one of the first variants of concern to emerge, leading to significant global spread. Studies have shown that the initial COVID-19 vaccines provided robust protection against this variant. For instance, a study conducted in the United States demonstrated that mRNA vaccines were highly effective in preventing hospital admissions due to the Alpha variant, with an effectiveness rate of 85% after two doses (Lauring et al., 2022). Similarly, a Danish nationwide cohort study reported a vaccine effectiveness (VE) of 90.7% against Alpha infection 14 to 30 days post-vaccination, which waned to 73.2% after more than 120 days (Gram et al., 2022). These findings underscore the high initial effectiveness of the vaccines against the Alpha variant, although a gradual decline in protection over time was observed. 3.2 Effectiveness against delta variant The Delta variant (B.1.617.2) posed a more significant challenge due to its higher transmissibility and partial immune escape capabilities. Vaccine effectiveness against the Delta variant was generally lower compared to the Alpha variant but still substantial. In the United States, mRNA vaccines showed an effectiveness of 85% against hospital admissions after two doses, which increased to 94% after a third dose (Lauring et al., 2022). A study in England found that vaccine effectiveness against symptomatic disease caused by the Delta variant was 65.5% after two doses of the BNT162b2 (Pfizer-BioNTech) vaccine, dropping to 8.8% at 25 or more weeks post-vaccination. However, a booster dose significantly increased protection, with effectiveness rising to 67.2% shortly after the booster (Andrews et al., 2022). In Denmark, the VE against Delta infection was 82.3% shortly after vaccination, decreasing to 50.0% after more than 120 days. The third dose substantially increased protection, with VE estimates of 86.1% against Delta infection 14 to 30 days post-vaccination (Figure 1) (Gram et al., 2022). These results highlight the importance of booster doses in maintaining high levels of protection against the Delta variant. Figure 1 Adjusted VE against COVID-19-related hospitalization after 2 or 3 doses of BNT162b2 mRNA or mRNA-1273 by SARS-CoV-2 variant and age group (Adopted from Gram et al., 2022) Image caption: Panel a represents VE against COVID-19 hospitalization after 2 doses. Panel b represents adjusted VE against COVID-19 hospitalization after 3 doses. The VE estimates are adjusted for underlying calendar time, age, sex, comorbidity, and geographical region. CI, confidence interval; COVID-19, Coronavirus Disease 2019; SARS-CoV-2, Severe Acute Respiratory Syndrome Coronavirus 2; VE, vaccine effectiveness (Adopted from Gram et al., 2022) 3.3 Effectiveness against Omicron variant The Omicron variant (B.1.1.529) has presented the most significant challenge to vaccine effectiveness due to its extensive mutations, particularly in the spike protein, which have led to increased immune evasion. Initial studies indicated that the primary vaccination series provided limited protection against Omicron infection. For example, a systematic review found that the protection from primary vaccination against Omicron infection was inferior to that against Delta and Alpha infections, with effectiveness waning faster over time (Paul et al., 2023). Another study reported that vaccine effectiveness against symptomatic disease caused by Omicron was only 65% shortly after two doses, dropping to 8.8% at 25 or more weeks (Andrews et al., 2022).
RkJQdWJsaXNoZXIy MjQ4ODYzNQ==