ONS survey data finds vaccines have reduced efficacy against the Delta variant of SARS-CoV-2 compared to the Alpha variant

Person of colour receiving vaccination_413207149_AS_Zoran Zeremski

The effectiveness of the Pfizer–BioNTech COVID-19 vaccine is reduced by up to 13% against the Delta variant of severe acute respiratory syndrome cornonavirus-2 (SARS-CoV-2) compared to the Alpha variant, and the AstraZeneca–Oxford University COVID-19 vaccine 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 from test-negative cases, 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, in which 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 vaccine effectiveness based on overall RT-PCR positivity, using cycle threshold (Ct) value (<30 versus ≥30) as a surrogate for viral load. The data were separated into two periods: from 1 December 2020 to 16 May 2021, when the Alpha variant dominated, and from 17 May 2021 to 1 August 2021, when the Alpha variant was replaced by the Delta variant.

The two vaccines provided similar benefits when the Alpha variant was dominant, but both showed reduced effectiveness against the Delta variant, with the effectiveness of two doses of the AstraZeneca–Oxford University vaccine reduced by a greater extent than two doses of the Pfizer–BioNTech COVID-19 vaccine—effectiveness against infections with symptoms or high viral burden was reduced with the Delta variant by an absolute difference of 10–13% for the Pfizer–BioNTech vaccine and 16% for AstraZeneca–Oxford University vaccine compared with the Alpha 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 the two vaccines, with the Pfizer–BioNTech vaccine showing greater initial effectiveness but faster declines in protection against high viral burden and symptomatic infection.

The researchers also found no evidence that effectiveness varied by dosing interval but said that protection was higher in vaccinated individuals after a prior infection and in younger adults.

Infections with the Delta variant that occurred 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 the Delta variant.

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

Pouwels KB, Pritchard E, Matthews PC, et al. 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.



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