Efficacy of Oxford/AZ and Pfizer vaccines against Delta variant in the UK

Simon Collins, HIV i-Base

Two papers from Public Health England (PHE), one ahead of peer review, have reported likely efficacy of the Oxford and Pfizer vaccines against the Delta variant.

The first, published in the NEJM, reported modestly reduced efficacy against symptomatic infection with both vaccines against Delta compared to Alpha variants. The absolute reductions were greater after a single dose which was slightly mitigated following the second dose. (See Table 1).

The second paper, not yet peer-reviewed reported similar efficacy against hospitalisation to both variants. (See Table 2).

However, this analysis was based on only 166 hospitalisations out of 14,019 symptomatic cases with Delta and may be underpowered to show differences.

Both studies report slightly higher absolute efficacy with the Pfizer compared to the Oxford vaccine.

Another recent paper reported lower sensitivity of both Oxford and Pfizer vaccines to the Delta variant with efficacy after two doses estimated at 60% and 88%, respectively. [3]

Table 1: Vaccine efficacy % (95%CI) against symptomatic infection (Lopez Bernal et al.)

Delta variant Alpha
Single dose (both Pfizer and Oxford) 30.7%

(25.2 to 35.7)


(5.5 to 51.7)

Two doses (Pfizer) 88.0%

(85.3 to 90.1)


(91.6 to 95.3)

Two doses (Oxford) 67.0%

(61.3 to 71.8)


(68.4 to 79.4)

Table 2: Vaccine efficacy % (95%CI) against hospitalisation (Stowe et al.)

Delta variant Alpha
Single dose (Pfizer) 94%

(46 to 99)


(62 to 93)

Two doses (Pfizer) 96%

(86 to 99)


(78 to 99)

Single dose (Oxford) 71%

(51 to 83)


(61 to 75)

Two doses (Oxford) 92%

(75 to 97)


(53 to 96)


  1. Lopez Bernal J et al. Effectiveness of COVID-19 vaccines against the B.1.617.2 (Delta) variant. NEJM. DOI: 10.1056/NEJMoa2108891. (21 July 2021).
  2. Stowe J et al. Vaccines highly effective against hospitalisation from Delta variant. PHE, pre-review access. (14 June 2021).
  3. Planas D et al. Reduced sensitivity of SARS-CoV-2 variant Delta to antibody neutralization. Nature. doi: 10.1038/s41586-021-03777-9. (29 June 2021).

This report was first published on 26 July 2021.

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