Two doses of both the Pfizer and the AstraZeneca COVID-19 vaccines were highly effective at preventing hospitalizations in those infected with the same variant of COVID-19 that has led to a surge in cases and forced the British government to postpone plans to remove remaining social distancing restrictions, an analysis of U.K. government data has shown.

The research—conducted by scientists with Public Health England, the National Health Service, and several leading British universities—indicated that two doses of AstraZeneca’s vaccine was 92% effective against hospitalizations among those infected with the Delta variant. It was also 100% effective at preventing deaths. But the same study showed that one dose of the vaccine was only 71% effective at preventing hospitalizations.

Two doses of the AstraZeneca vaccine were about 64% effective at preventing people from developing mild to moderate COVID-19 symptoms, the study indicated.

Meanwhile, the Pfizer vaccine was 94% effective at preventing hospitalizations from the Delta vaccine after just one dose and 96% effective after two. It too was 100% effective at preventing deaths. And the vaccine was about 88% effective at preventing any symptoms after two doses.

The analysis of data from Public Health England has contributed to government decisions to accelerate vaccination efforts and delay plans to lift several key social distancing measures, including mask requirements and those aimed at preventing large, crowded gatherings, that remain in place.

The Delta variant is formally known as variant B.1.167.2, and is sometimes referred to as “the Indian variant” because it was first reported in that country and was responsible for the devastating second wave of infections there. This strain, which is highly transmissible, has been outcompeting earlier strains of the coronavirus, including the variant designated as B.1.1.7 and referred to variously as the Alpha, Kent, or U.K. variant. That strain had become the dominant variant in the U.K. since first being identified in September 2020. But the newer Delta variant now accounts for nine out of every 10 new COVID-19 cases diagnosed.

It has led to a sudden spike in infections. The seven-day moving average of new cases has climbed to more than 7,200 as of Monday, more than three times the level it had been one month earlier.

Those numbers, along with projections from disease modelers that the number of hospital admissions would equal those seen in the first wave of the pandemic in spring 2020 if U.K. Prime Minister Boris Johnson went ahead with plans to end all legally required social distancing measures by June 21, forced Johnson to announce a four-week delay to the lifting of those restrictions.

Johnson said the delay would allow more time for people over 40, who are those most likely to become seriously ill with COVID-19, to be fully vaccinated before social distancing measures are completely lifted.

The evidence of the vaccine’s effectiveness against the Delta variant has also forced the British government to rethink how it spaces out doses. At the start of this year, the government gambled on a strategy of using its available vaccine supply to give as many people as possible first doses of a vaccine, based on evidence that even one dose provided substantial protection against hospitalization and death. In order to do that, however, it had to stretch out the gap between doses to 12 weeks, far beyond the three or four weeks that had been tested in the initial clinical trials for the Pfizer and AstraZeneca vaccines. (Later studies showed the AstraZeneca vaccine seemed to become even more effective with a 10- to 12-week interval between doses.)

Now, in order to get more of the population the two doses needed to provide better protection against the Delta variant, the government is considering reducing the dosing interval to eight weeks for those over 40 years old. The Scottish government officially made this decision yesterday while the English health authorities are still debating it.

About 56% of the U.K.’s adult population has now been fully vaccinated against the coronavirus, while about 78% have received at least one dose of a vaccine. But the figures vary by region, age group, and ethnicity. Only about 34% of London-area adults are fully vaccinated so far, for instance. And vaccination rates among Black Britons are lagging substantially behind those who are white, across all age groups. For instance, 68% of Black people ages 60 to 64 have been vaccinated, while 93% of whites in the same age bracket have had at least one jab.