A Norfolk virologist has said we will not fully understand how the Omicron variant is spreading until the end of next week.
It comes as the health secretary Sajid Javid confirmed no new Covid restrictions will be introduced before the New Year.
But Mr Javid said people should "remain cautious", take a lateral flow test before socialising and celebrate New Year's Eve outside if possible - or have ventilation indoors if not.
Virus expert Prof Paul Hunter, from the University of East Anglia's School of Medicine, said it was difficult to draw many conclusions from data at this stage.
In the days leading up to Christmas, he had said he was optimistic that data suggested Covid-19 cases were plateauing and that Omicron was not growing exponentially.
But he said data over the Christmas period would be very difficult to make sense of for a number of reasons.
He said: "There's an expectation that the latest figures are going to be really insightful, but that is probably approaching being delusional.
"That's for a number of reasons, but in my 40-year career, interpreting figures around Bank Holidays - and Christmas - is a nightmare."
He said people were probably less likely to take and submit test results in the run-up to Christmas and on Christmas Eve, Christmas Day and Boxing Day itself, while there was also a delay in processing samples over the Christmas period.
He added that the difficulties people experienced in getting testing kits in the days before Christmas could also skew the statistics.
He said: "Afterwards you get a bit of a rebound, but some people still won't have put in test results, while there is also a delay in processing.
"I would be quite happy if the latest figures showed cases stayed flat. But the only thing which would really tell us much with any certainty is if cases go through the roof. If that has happened, then we are clearly in trouble."
Prof Hunter said it would not be until towards the end of next week that the data becomes useful.
He said: "There's some quite reassuring data and some quite scary data and trying to figure out the balance is very difficult.
"Is it better to have intense pressure for a couple of weeks, or is it better to have less pressure, but over two months?
"You go to doctors and nurses and there are, quite rightly, really worried about that peak, whereas much of the rest of the country might like it to be got over more quickly so their business is not curtailed.
"But there's also mental health issues to consider and that children are at more risk of abuse when restrictions which keep them at home are introduced."
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