The UK is anticipating a dip in its coronavirus vaccine supply in the coming weeks, with potential to disrupt inoculation targets.

Financial Times analysis suggests that, if the current rate is maintained, the government’s latest inoculation goal could be achieved by the end of March.

After hitting its mid-February goal for offering vaccines to the 15m most vulnerable UK residents, the government is aiming to give jabs to the next 17m most vulnerable, including all over-50s, by April 30. Vaccinating this group is critical for reducing pressure on the NHS and easing the nationwide lockdown.

The UK has delivered close to 3m doses a week over the past fortnight. If that supply and distribution level is maintained, the FT analysis suggested that its next target could be met by the week ending March 28.

Senior officials have spoken privately of the end of March as the anticipated date for completing the first phase of the programme.

First doses are now being offered in large numbers to those aged between 60 and 70. But by early March, 12 weeks will have passed since vaccination of the government’s top-four priority groups began, meaning that supplies will need to be reserved for these people’s second doses.

Because of the relatively low numbers that were inoculated until early January, however, the number of second doses required each week will remain well below 3m for the next two months, giving plenty of headroom for first doses to continue rolling out to the next five priority groups.

The government’s much longer target of April 30 suggests that ministers either expect supply levels to drop in the coming weeks, or want to play down expectations, stung by the criticism they have suffered during the pandemic when stated deadlines have not been met.

According to individuals familiar with the programme, supply is expected to remain fairly constant over the next few months but some blips have been factored into planning.

One senior Number 10 official admitted that vaccine supply was “a difficult one” and the government was cautious on whether supplies would continue at the almost 3m doses a week rate seen in early February. “Targets for the end of March do not reflect our internal assessment,” the individual said.

Chart showing that at current pace, the UK will have given a first Covid-19 vaccine dose to all over-50s by the end of March while also meeting commitments to deliver second doses to older groups

Health department insiders said they were “confident of supply to hit our target for the end of April”.

But others across Whitehall think that the April 30 target is an expectations management strategy, part of Downing Street’s new cautious approach to under-promise and over-deliver.

Mark Drakeford, Wales’s first minister, confirmed last week that supplies were expected to decline over the next few weeks. The principality hopes that deliveries will start rising again by March.

Nicola Sturgeon, Scotland’s first minister, also on Tuesday acknowledged that delays would affect her administration’s vaccine rollout in the coming weeks. While Pfizer had not reduced the volumes it was sending to the UK, it had “rephased” its delivery schedule, she said.

She told the Holyrood parliament: “Over the next couple of weeks we are getting slightly less in supply than we thought originally we would be getting.” The rest of the UK is expected to be affected by similar constraints, people briefed on the situation said.

The government also on Tuesday advised 1.7m more people to shield, taking the total to 4m, after modelling technology identified more risk factors for developing Covid-19. QCovid, a risk assessment tool, uses a range of considerations, such as previous medical history, to determine who may be at higher risk of contracting coronavirus.

The government’s New and Emerging Respiratory Virus Threats Advisory Group developed the technology with researchers from Oxford university and discovered that factors including age, weight and ethnicity could increase the risk of developing Covid-19.

Meanwhile, the first scientific evidence that England’s vaccination programme was having an effect was published on Tuesday. The Office for National Statistics reported a substantial increase in numbers testing positive for coronavirus antibodies, with the largest rise in people aged over 80.

The ONS said the proportion of people in England with detectable antibodies in their blood in the 28 days to February 1 was one in five. The comparable figure released two weeks ago was one in seven.

Although some observers claim to have seen an effect, with cases and hospitalisation rates falling more quickly among the elderly than younger people, a senior government adviser said: “We have not seen a clear signal yet. Distinguishing a signal from the noise is not straightforward.”

A further complication emerged on Tuesday with news that another Covid-19 variant called B.1.525, first found in Nigeria in December, had been detected in the UK. It includes the E484K mutation, also present in variants from South Africa and Brazil, which are believed to help the virus evade some of the body’s immune defences.

Additional reporting by Jasmine Cameron-Chileshe