Does he act now and impose harsher restrictions for England, despite not having the full picture about the threat posed by this latest wave of COVID?
Or does he wait for more information, in the hope that the outlook is not as gloomy as many scientists predict?
For now, the answer coming out of Downing Street is to wait, at least for a bit longer.
“We don’t know enough yet about how serious this is going to be… we need to see the impact of Plan B and boosters,” said one Whitehall source.
That is a risk, though.
Government scientists are warning that holding back at a time when Omicron infections are doubling every two days will “bake in” a certain number of hospitalisations, as some of those who are infected now begin to get seriously ill by the end of the year.
How many, though, is the big unknown.
The risk for the prime minister is that by the time he has that key piece of information, the die will already have been cast and a chain of events set in motion that could put the NHS under unsustainable pressure.
Then there’s the political dimension to all this.
Boris Johnson has already lost a cabinet minister, Lord Frost, in part because of the existing Plan B set of restrictions.
One Conservative MP said the reaction among backbenchers to any cranking up of regulations would be “pretty nuclear”.
There would also be huge pressure for the PM to recall parliament to scrutinise proposals.
There’s suspicion too among some MPs at what they see as characteristically gloomy assessments from scientists.
Former chief whip Mark Harper said the models were “garbage in, garbage out – because the assumptions are not based on evidence”.
“The modelling ahead of us opening up was all way too pessimistic”, said the Tory MP.
It doesn’t help that trust in Boris Johnson from within his party is a dwindling currency at the moment.
Hobbled by a thumping loss in the North Shropshire by-election and dogged by scandals that seem unwilling to shift, the prime minister is hardly in a position to drive through politically unpalatable changes.
That’s perhaps why talk of a circuit breaker period of restrictions were heavily played down today, with one source saying that “wouldn’t be the kind of thing we’d look at”.
All that said, the cogs of government do appear to be turning this weekend, with the cabinet briefed on the latest COVID data and a COBR called for Sunday.
That meeting will also involve the devolved administrations.
The announcement of restrictions in Wales after Christmas and the indication from Scotland that more measures may be needed will only increase the pressure on the Westminster government to act.
At the most vulnerable point of his premiership, Boris Johnson is once again facing a huge decision that will shape the lives of many and potentially dictate the future smooth running of the NHS.