Covid variants: how much protection do the different vaccines offer?

On Wednesday Boris Johnson said he saw nothing in the current data to stop the planned lifting of Covid restrictions in England on 21 June. But he said questions remained over how much protection the current vaccines offered against the Delta variant, B.1.617.2.

What does vaccine effectiveness mean?

Vaccine effectiveness is given as a percentage and refers to how well a vaccine works in the real world, as opposed to the term efficacy, which relates to vaccines’ performance in trials. The figures are often given with respect to symptomatic disease, although other outcomes, such as infection, hospitalisation or death, can be used.

If a Covid-19 vaccine is “90% effective” against symptomatic disease that means the risk of becoming ill is 90% lower among vaccinated people than among those who have not been vaccinated. For every 100 unvaccinated people who ended up with symptomatic Covid, for instance, only 10 would have been expected to have fallen ill had they received the jab.

What is vaccine failure?

The term is often used to refer to cases where people develop an infection or disease despite being vaccinated against it. This can relate to a problem with the vaccine itself, but can also refer to some people having a weaker immune response to the vaccine.

Using symptomatic disease as the outcome, a Covid vaccine that is “90% effective” could be said to have a 10% failure rate (ignoring changes in effectiveness over time). However, someone who is not protected against symptoms could still be protected against severe disease or death.

Deborah Dunn-Walters, a professor of immunology at the University of Surrey, says: “If you think of the population having a whole range of different levels of immunity, then where you set the bar in the range will determine the percentage protection figures.”

How effective are the vaccines against the Delta variant, B.1.617.2, compared with the Alpha variant, B.1.1.7 ?

Some studies, including lab-based work and analysis of real-world data, have suggested current Covid vaccines are somewhat less effective against the Delta variant than the Alpha variant.

According to Technical Briefing 13 from Public Health England, an analysis involving 7,673 symptomatic cases identified as B.1.1.7 and 2,934 cases identified as B.1.617.2 revealed that after a single dose of the Pfizer/BioNTech or Oxford/AstraZeneca Covid vaccines “there was a 17% absolute reduction in vaccine effectiveness against symptomatic disease with B.1.617.2 compared to B.1.1.7, but only a modest reduction in vaccine effectiveness after two doses”.

The take-home message, says Dunn-Walters, is that “vaccines protect, and a second dose is very important against the Delta variant”.

What does 17% reduction mean?

The 17% reduction in vaccine effectiveness relates to symptomatic disease after one dose. But it does not tell us about disease severity. That matters because if the vaccines offer lower protection against severe disease from the Delta variant, the burden on hospitals as cases rise will be greater than otherwise.

Prof Adam Finn of Bristol University, who is a member of the Joint Committee on Vaccination and Immunisation, says: “We don’t have certainty [over] the impact on vaccine effectiveness, especially regarding protection versus severe disease after one dose and after two doses.” It is also unclear how much more infectious the Delta variant is, he says.

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.

What could this mean for a third wave?

Finn says: “A more infectious virus which is able to escape vaccine induced immunity more often will cause a faster rise in cases as people mix more. But if the vaccines retain their ability to prevent severe disease then hospitalisations will not rise as fast as they have previously.”

Experts have said it is a “realistic possibility” that the Delta variant is as much as 50% more transmissible than the Alpha variant, while modelling suggests a variant that is 40% more transmissible than the Alpha variant could lead to daily hospitalisations exceeding those recorded over the winter. That is assuming the planned relaxation of Covid social restrictions goes ahead in June – and without taking into account any resistance of the variant to the vaccines.

When we will know more?

The picture is likely to take time to become clearer not least because while cases are rising numbers remain low, meaning a longer period is needed to compare outcomes for hospitalisation and other measures among vaccinated, partially vaccinated and unvaccinated people.

“We may not get good answers for a matter of weeks, or even months,” said Dr Peter English, a retired consultant in communicable disease control.