Boris Johnson this week unusually declined an opportunity to engage in boosterism. At a Downing Street press conference the prime minister refused to cheer on the UK’s fight against Covid-19, instead speaking of “a cautious, pragmatic approach that we won’t have to retreat from”.

The shift in rhetoric is notable for a politician who built his reputation on buoyant optimism. Whereas Johnson spent much of 2020 pledging a rapid return to normality from the pandemic-induced restrictions, 2021 has seen a much less gung-ho attitude.

Across Whitehall, two factors are cited in Johnson’s reticence towards easing England’s third nationwide lockdown. One is Covid-19 itself: the spread of new strains, some of which may prove more vaccine resistant. The other is Number 10’s eagerness for this to be the last lockdown.

One senior government official said: “This has to be the last lockdown, no doubt about it. The PM is clear on that. But with the new strains, there’s a real concern that we can’t open up as quickly as we would like.”

“What we can’t do is give people back some freedoms and then watch the virus take off and have to take them away again,” one close ally of Johnson said. “It’s been an extraordinarily difficult period and that would be a real kick in the teeth.”

Another individual closely involved in Covid-19 planning said: “We are constantly torn between over and under promising. It’s going to be a rough few weeks when the numbers go down. That will be the real challenge, can we hold the newfound message discipline?”

Johnson’s caution is likely to be tested by libertarian Conservative MPs, especially the Covid Recovery Group of lockdown sceptics. Mark Harper, chair of the caucus, has said that all restrictions should be lifted as soon as the most vulnerable are jabbed. One government official acknowledged: “We’re not dealing with rebellious backbenchers well enough.”

Another factor in Johnson’s new demeanour is the shake-up of his Downing Street team. Following November’s departure of chief aide Dominic Cummings and other advisers associated with the Brexit referendum campaign, multiple government insiders said the new operation had reshaped the prime minister’s political stance.

One senior Tory figure said: “Sitting at the heart of this, you have the mercurial personality of the PM. He hasn’t changed, but events and the people around him have instilled in him a greater degree of being careful.”

A senior civil servant said “calmer” was the first word that came to mind in describing Number 10. “At the height of the old regime, everything was frantic and we were lurching from one thing to another. Now it feels as if we’re on a path to being slightly more strategic and considered.”

Dan Rosenfield, who started as Number 10 chief of staff in January, has brought “a greater sense of organisation” to government, according to those working inside Downing Street. Rosenfield is a “stand-up desk, corporate machine type of guy”, who does not appear to have his own political agenda, one insider said. “It’s a good thing that he’s there to just do what the PM wants.”

Another senior official said things were “less capricious” since the departure of Cummings and his allies. “There is more policy grip and that appears to be down to the people around [Johnson]. We no longer have senior people who previously stirred the pot. It feels more grown up.”

The influence of the civil service “machine” has grown in Johnson’s inner team over the past two months, notably with the appointment of Sir Stephen Lovegrove, the Ministry of Defence’s permanent secretary, as his new national security adviser. “It’s a really big signal that the machine is grabbing the political part of government by the balls,” one Whitehall insider said.

Allegra Stratton, Johnson’s press secretary, is also said to have had a significant influence on tempering the prime minister’s natural instincts. She has told colleagues that their motto is “we need to be sensible and calm, not be carried away”.

Stratton joined Number 10 in October to host daily press conferences, which were set to begin in January but have been delayed due to the pandemic: advisers believe that the nation needs reassurance from the prime minister and ministers while in lockdown.

Along with James Slack, director of communications, and his deputy Jack Doyle, the press operation is “significantly less chaotic”, according to Whitehall insiders, although one Tory party official cautioned: “There are still too many people going off script.”

The latest row about whether Britons will be allowed to go on holiday at all this year has suggested there is still confusion among ministers about how to temper optimism about the months ahead.

Some senior Whitehall figures think the government had yet to find the right balance in managing public expectations as the third lockdown, combined with a cold snap across the UK, begins to grind down the country.

“We still need to find the sweet sailing spot: close to the wind, without turning into it,” an official said. “How do we keep the lid on everything while reminding people things will get better?”