The first rule of world-beating vaccine club is you don’t talk about world-beating vaccine club.
A weird one, especially when the first rule of world-beating coronavirus test and trace club seemed to be that you talk about little else. Even when the reality is clearly not living up to the description.
So what’s going on?
Some of this can be explained by recent personnel changes in Downing Street and a more nuanced approach to communication strategy.
Less over-promising, more over-delivering.
But Boris Johnson also knows that too much trumpeting of vaccine triumphs increases the expectation of a more rapid retreat from lockdown.
Indeed, noises from the Conservative backbenches for a bonfire of COVID-19 regulations are growing.
As is the frustration that the one-time optimist-in-chief in Number 10 is now a good deal more dour.
The calculation in government though is that the political damage that would be done by bodging this final exit from the pandemic is so great that it can weather accusations of lumpen glumness.
No one wants to see a “reverse ferret” after restrictions are eased this time, as the PM put it on Saturday.
What’s more; despite his newly measured tone, the vaccine success story has still given Boris Johnson some fresh political capital that he can spend on a more cautious unrolling of lockdown.
That’s not to say this is just a question of political risk though.
The situation on the ground is still precarious with several thousand more people in hospital with COVID than at the first peak last year.
Scientists advising the government are calling for a step-by-step approach, stopping at each stage to measure any increase in infections.
While this does appear to be a guiding principle in Whitehall, on schools the ambition is bolder.
The aim as it stands is not for a phased return, but to get all pupils back in the classroom from 8 March.
Such a snap-back could have repercussions though, potentially pushing up the reproduction rate of the virus and slowing down easing in the rest of society.
Again though, that is a risk that Downing Street can afford to take.
What might change the calculation and give Boris Johnson more bandwidth for optimism is a pack of figures that will land on his desk this week.
Public Health England is currently looking at the first tranche of real-world data on the effectiveness of the vaccine in the UK and crucially how much it reduces transmission.
There is “cautious optimism” it will tell a good story and point to an expected drop in hospitalisations, deaths and cases.
It is that data that will determine how far the PM can go when he reveals his route out of lockdown in a week’s time.
But it’s that data that will also provide ammunition for those who want Boris Johnson to shake off his new found caution and starting talking more about world-beating vaccine club.