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How are the vaccines affected by the Delta variant?

20 August 2021

8:03 AM

20 August 2021

8:03 AM

Has the emergence of the Delta, or Indian, variant reduced the effectiveness of Covid vaccines and if so by how much? The fear that the Delta variant might be partially escaping our vaccines has often been expressed in recent weeks, especially given that highly-vaccinated countries such as Israel and parts of the US have suffered new peaks of infection since the Delta variant came to dominate. But there has been a lack of real-world data to prove whether or not there is any grounding to these fears.

Today, however, the Oxford University Covid 19 Infection Survey publisheda study which seeks to answer this question. It compared the results of PCR tests on over 700,000 people, around half of whom were swabbed between December 2020 and May 2021 when the Alpha (Kent) variant was dominant, and half of whom were swabbed between May and August 2021 when the Delta variant was dominant.


The study concludes that yes, there is a drop in effectiveness, but not by all that much. As in trials, Pfizer was found to be initially more effective than AstraZeneca, and its effectiveness against the Delta variant held up slightly better. Among those given the AstraZeneca jab, vaccine effectiveness (the protection against symptomatic infection) fell from 75 to 67 per cent. Among those given Pfizer, effectiveness fell from 94 to 88 per cent. The study also looked at the protective effect over time and concluded that the effectiveness of the Pfizer vaccine has been falling by 22 per cent per month and the effectiveness of AstraZeneca by 7 per cent per month. Through extrapolation (as opposed to real world data) the team concluded that the effectiveness of Pfizer and AstraZeneca would be similar around four to five months after vaccination.

The study also found that people infected after being inoculated with either vaccine had higher viral loads during the period when the Delta variant was dominant compared with the earlier Alpha period. Indeed, during the Delta-dominant period viral loads among infected, vaccinated people were as high as among infected, non-vaccinated people. This suggests that it is harder for vaccines to prevent the transmission of the Delta variant than it is for them to prevent transmission of the Alpha variant. This does not mean that vaccines have no effect on transmission, as vaccinated individuals were less likely to become infected in the first place.

As ever there are some caveats. As the study progressed it became harder and harder for the study to identify unvaccinated individuals among older groups. For that reason, the study does not offer data on those aged 65 and over. Moreover, the study did not individually test every infected individual to see what variant they had been infected with – it simply compared infections during two periods, one where the Alpha variant was dominant and one where the Delta variant had taken over.

But it will provide some comfort against fears that the Delta variant is escaping the vaccines – although the general decline in vaccine effectiveness over time will strengthen the case for booster jabs which the government is planning for the over-50s from early September.

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