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The Covid Delta variant: how effective are the vaccines?

The Covid Delta variant: how effective are the vaccines?

As lockdown easing in England is delayed from 21 June to a possible date of 19 July amid concerns of a substantial wave of hospitalisations due to the Delta variant of coronavirus, we take a look at the latest data on the protection offered by vaccines.

What does vaccine effectiveness mean?

Vaccine effectiveness refers to how well a vaccine works in the real world. The figures are given with respect to particular outcomes, such as infection, symptomatic disease, hospital admission or death.

If a Covid-19 vaccine is “90% effective” against symptomatic disease it means vaccinated people have a 90% lower risk of developing the illness than those who have not been vaccinated. That means that for 100 unvaccinated people who developed symptomatic Covid, only 10 would have been expected to have fallen ill had they been vaccinated.

However, it important to note that vaccine effectiveness figures are estimates that lie within a range of possible values.

The Delta variant, first detected in India – which now accounts for more than 90% of new Covid cases in the UK – has raised concerns as it appears to be somewhat more resistant to vaccines than the Alpha variant that was first detected in Kent and previously dominated in the UK.

How effective are the vaccines against the Delta variant compared with the Alpha variant?

In general, Covid jabs are most effective against the most severe outcomes, such as death, and less effective against less severe ones, such as asymptomatic infection.

This appears to be borne out by data collected for the two main Covid jabs in use in the UK against both variants.

According to figures gathered by Public Health Scotland and published in the Lancet, at least two weeks after the second dose of Covid jabs, protection against infection fell from 92% for the Alpha variant to 79% against the Delta variant for the Pfizer/BioNTech vaccine, while for the Oxford/AstraZeneca vaccine the protection fell from 73% to 60% respectively.

Protection against symptomatic disease has also been found to differ depending on the variant. According to early work by Public Health England (PHE), one dose of either jab offered just over 51% protection against the Alpha variant, compared with 33.5% protection against the Delta variant – figures that changed very little in further work. After two doses the differences were more modest. With the Pfizer/BioNTech jab protection against the Alpha variant was 93.4%, compared with 88% for the Delta variant. For the Oxford/AstraZeneca vaccine, protection fell from about 66% to 60% – although experts say there are a number of reasons why this jab has lower figures for either variant, including that it takes longer to reach maximum effectiveness.

The most recent figures from PHE suggest that after two doses, the Pfizer/BioNTech jab offered 88% protection against symptomatic disease with the Delta variant, compared with 67% for two doses of the Oxford/AstraZeneca vaccine.

What about hospital admissions?

Protection against severe disease is a crucial metric, not least for assessing the potential pressures a new wave of infections could place on health services. The good news is that for this outcome the jabs appear to offer similar protection against both variants.

According to an analysis by PHE the Pfizer/BioNTech jab was linked to a 94% vaccine effectiveness against hospital admission with the Delta variant after one dose and 96% after two doses, while the figures for the Oxford/AstraZeneca jab were 71% and 92% respectively.

This protection against hospital admission is similar to that for the Alpha variant for which the Pfizer figures were 83% and 95% after the first and second jabs respectively, and 76% and 86% respectively for the AstraZeneca jab.

What about deaths?

At present there is not enough data to say, but the results are expected to be promising. According to PHE: “Further work remains under way to establish the level of protection against mortality from the Delta variant. However, as with other variants, this is expected to be high.”


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