BACKGROUND AND PURPOSE: 

• The Delta SARS-CoV-2 variant (B.1.617.2) has been detected across the globe, and is quickly becoming the dominant variant  
• Bernal et al. (NEJM, 2021) estimated the effectiveness of the Pfizer and AstraZeneca vaccines against the delta variant 

METHODS: 

• Test-negative case-control study 
• Population 
  • All symptomatic sequenced cases of COVID-19 in England 
  • Vaccination data up through May 16, 2021 
• Exposures 
  • Delta variant 
  • Predominant strain at time of study (B.1.1.7 or alpha variant) 
• Study design 
  • Variants were identified using sequencing and on the basis of the spike (S) gene status 
• Primary outcome 
  • Proportion of cases with either variant according to vaccination status 

RESULTS: 

• 38,592 cases with linked sequencing data 

After 1 Dose  

• Effectiveness of one dose of either Pfizer of AstraZeneca  
  • Against alpha: 48.7% (95% CI, 45.5 to 51.7) 
  • Against delta: 30.7% (95% CI, 25.2 to 35.7) 
• Results were similar for one dose of either vaccine individually 

After 2 Doses 

• Effectiveness of two Pfizer doses  
  • Against alpha: 93.7% (95% CI, 91.6 to 95.3) 
  • Against delta: 88.0% (95% CI, 85.3 to 90.1) 
• Effectiveness of two AstraZeneca doses  
  • Against alpha: 74.5% (95% CI, 68.4 to 79.4) 
  • Against delta: 67.0% (95% CI, 61.3 to 71.8) 

CONCLUSION: 

• The effectiveness of two doses of both Pfizer and AstraZeneca’s COVID-19 vaccines were only slightly lower against the delta variant 
• There was a more marked difference in effectiveness if individuals had received only one dose 
• The authors conclude 

Our finding of reduced effectiveness after the first dose would support efforts to maximize vaccine uptake with two doses among vulnerable groups in the context of circulation of the delta variant 

Bernal et al. (NEJM, 2021)

Learn More – Primary Sources

Effectiveness of Covid-19 Vaccines against the B.1.617.2 (Delta) Variant 

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Efficacy of the Novavax SARS-CoV-2 Vaccine Against UK and South African Variants

The Value of Vaccination for Those Previously Infected with SARS-CoV-2

Safety and Efficacy Data of BNT162b2 COVID-19 Vaccine

Safety and Efficacy of AstraZeneca Oxford’s COVID-19 Vaccine

COVID-19 mRNA Vaccine Effectiveness in the Real World Including in Those Partially Immunized