COVID-19: Vaccines up to 16% Less Effective Against Delta

Dawn O'Shea

October 28, 2021

The effectiveness of the Pfizer/BioNTech COVID-19 vaccine (BNT162b2) is reduced by up to 13% against the Delta variant (B.1.617.2) of SARS-CoV-2 compared with the Alpha variant (B.1.1.7), whereas the AstraZeneca/Oxford vaccine (ChAdOx1) is 16% less effective against Delta, according to an analysis of data from the Office for National Statistics (ONS) COVID-19 Infection Survey (CIS).

In contrast to previous studies which have estimated vaccine effectiveness (VE) from test-negative case, which may have been biased by test-seeking behaviour of cases not requiring health care, this study, published in Nature Medicine,  assessed the effectiveness of vaccines against new polymerase chain reaction (PCR)-positive cases in the large, community-based CIS survey of individuals living in randomly selected private households across the UK, where reverse transcription (RT)-PCR tests were performed after a pre-determined schedule, irrespective of symptoms, vaccination and prior infection.

The research team, led by Oxford University, assessed VE based on overall RT-PCR positivity, using cycle threshold (Ct) value (<30 vs ≥30) as a surrogate for viral load. The data were separated into two periods: from 1 December 2020 to 16 May 2021, when B.1.1.7 dominated, and from 17 May 2021 to 1 August 2021, when B.1.1.7 was replaced by B.1.617.2.

The two vaccines provided similar benefits when B.1.1.7 was dominant, but both showed reduced effectiveness against the Delta variant. The effectiveness of two doses of ChAdOx1 was reduced more than for two doses of BNT162b2.

The effectiveness of BNT162b2 and ChAdOx1 against infections with symptoms or high viral burden was reduced with the B.1.617.2 variant (absolute difference of 10%-13% for BNT162b2 and 16% for ChAdOx1) compared with the B.1.1.7 variant. However, the effectiveness of two doses remained at least as great as protection afforded by prior natural infection.

The dynamics of immunity after second doses differed significantly between BNT162b2 and ChAdOx1, with BNT162b2 showing greater initial effectiveness but faster declines in protection against high viral burden and symptomatic infection.

There was no evidence that effectiveness varied by dosing interval, but protection was higher in vaccinated individuals after a prior infection and in younger adults.

With B.1.617.2, infections occurring after two vaccinations had similar peak viral burden as those in unvaccinated individuals.

The authors concluded that vaccination continues to reduce new infections, but caution that the effectiveness and attenuation of peak viral burden are reduced with B.1.617.2.

Pouwels KB, Pritchard E, Matthews PC, Stoesser N, Eyre DW, Vihta KD, House T, Hay J, Bell JI, Newton JN, Farrar J, Crook D, Cook D, Rourke E, Studley R, Peto TEA, Diamond I, Walker AS. Effect of Delta variant on viral burden and vaccine effectiveness against new SARS-CoV-2 infections in the UK. Nat Med. 2021 Oct 14 [Epub ahead of print]. doi: 10.1038/s41591-021-01548-7. PMID: 34650248

This article originally appeared on Univadis, part of the Medscape Professional Network.

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