Signs exist that a single dose of the BioNTech/Pfizer vaccine offers good protection against Covid-19, according to official data on the UK’s vaccination campaign, in further validation of the government’s approach taken to extend the gap between jabs.

Although there is not enough evidence to draw definitive conclusions about the impact of the vaccination campaign on deaths and hospitalisations in the UK, several people with access to government data said there were indications it was reducing cases in the groups which have been prioritised to receive the jab.

The initial analysis will come as a boost to ministers after there were concerns that the government’s decision to extend the gap between first and second jabs of the vaccines from three to 12 weeks could weaken the recipients’ immune response.

The rationale behind the decision was to ensure the maximum number of people got their first shots in the initial wave of the programme from the available supply of vaccines.

Just over 13m people in the most vulnerable categories in the UK have had their first dose so far after 411,812 received jabs on Tuesday, according to the latest official figures.

Boris Johnson on Wednesday said ministers would set out plans next week for the next stage of the programme, aimed at vaccinating a further 17m people.

Speaking at a Downing Street press conference, the UK prime minister urged those in the most vulnerable categories, who had not yet had a jab to do so as it would help speed up the end of lockdown. “There’s no doubt we have made great strides,” he said.

The efficacy data on the vaccination programme showed that it appeared to be triggering a good immune response about two weeks after the jab in younger adults — predominantly healthcare workers — and after about three weeks in the elderly, according to the people.

Data from Israel, the country leading the world in vaccinations, backs up the initial indications that the UK’s decision to extend the gap between jabs has paid off in terms of first-shot immunity.

Almost all of the data analysed relate to the BioNTech/Pfizer vaccine as it started to be administered three weeks before the Oxford/AstraZeneca jab.

Chart showing that although the vaccine is reported to be performing well after one dose, the results are yet to show up in national-level data, where cases and hospital admissions are not yet falling any faster in older groups than in younger groups

The Sun newspaper reported on Tuesday that Public Health England found that a single dose reduced symptomatic infection by 65 per cent in younger adults and 64 per cent in the over-80s, while two jabs conferred 79 and 84 per cent protection respectively.

However, one senior scientist who has seen the data, said it was “misleading to the public to give exact figures at the moment” as the numbers were changing daily and the confidence intervals were too wide.

But he added: “It is looking promising. The delayed second dose strategy is on track at the moment.”

He also noted it was very difficult to determine the vaccine’s effect on hospitalisations and deaths.

The PHE findings add to evidence from Israel, released last month, which showed that a single shot of the BioNTech/Pfizer vaccine produced a robust antibody response within weeks.

At the Rambam Health Care Campus in north Israel, 91 per cent of the 1,800 doctors and nurses who received the two-dose vaccine showed a major presence of antibodies 21 days after their first shot and before the second.

At the Sheba Medical Center, similar serological tests at different intervals showed at least 50 per cent of staff with a level of antibodies “above the cut-off point” two weeks after the first jab.

PHE is using a different method to determine whether or not the vaccine is having an impact on case rates in the population, cross-checking Covid-19 testing data with centrally held national vaccination records.

A PHE spokesperson said the body had been analysing the data since the start of the vaccination programme rollout and would publish its findings in due course.