New modelling for the government’s Sage committee of experts has highlighted the risk of a “substantial third wave” of infections and hospitalisations, casting doubt on whether the next stage of Boris Johnson’s Covid roadmap can go ahead as planned on 21 June. Government sources suggested the outlook was now more pessimistic but stressed that a decision would be taken after assessing a few more days’ worth of data on the effect that rising infections are having on hospitalisations. The prime minister is due to announce on Monday whether the lifting of the remaining restrictions – nicknamed “freedom day” by anti-lockdown Tory MPs – will have to be delayed. Johnson is understood to be personally frustrated at the prospect of delaying the reopening, but a No 10 source said there were now clearly signs for concern in the data. Key ministers and officials are expected to discuss a range of options on Sunday, when Johnson will still be hosting the G7, including a two- to four-week delay, as well as the possibility of a watered-down reopening that keeps some rules in place. A Whitehall source said it was “broadly correct” that the outlook was now more pessimistic. “Cases are obviously higher and they are growing quickly,” the source said. Prof Neil Ferguson, of Imperial College London, said modelling updated this week suggested there was a risk of a surge in infections and hospitalisations that could rival the second wave in January. Johnson sounded markedly less confident than in recent days when he was asked about the case for a delay as he visited a windfarm in Cornwall on Wednesday as part of the buildup to the G7 summit. “What everyone can see very clearly is that cases are going up and in some cases hospitalisations are going up,” he said. “I think what we need to assess is the extent to which the vaccine rollout, which has been phenomenal, has built up enough protection in the population in order for us to go ahead to the next stage. “And so that’s what we’ll be looking at. And there are arguments being made one way or the other, but that will be driven by the data. We’ll be looking at that and we’ll be setting it out on Monday.” The prime minister had previously repeatedly said he had seen nothing in the data to justify a delay. Ferguson said the cases of the Delta variant were now doubling in less than a week, close to what was seen before Christmas when the Alpha variant took hold and sent infections soaring in January to a daily peak of 68,000. What is unclear is how long the doubling will continue with so many adults vaccinated, and what proportion of new cases will turn into hospitalisations and deaths. “There is a risk of a substantial third wave,” Ferguson said. “It could be substantially lower than the second wave or it could be of the same order of magnitude, and that critically depends on how effective the vaccines are at protecting people against hospitalisation and death.” He suggested there may be a case for postponing the reopening to get more shots into arms and reduce the size of any summer surge. “Clearly you have to be more cautious if you want measures to be irreversibly changed and relaxed,” he said. “Having a delay does make a difference. It allows more people to get second doses.” Ministers have been encouraged by the enthusiasm with which younger people are taking up the opportunity to get their jab. The NHS announced that 1 million people had booked appointments through its website on Tuesday as eligibility was extended to 25- to 29-year-olds. The next two to three weeks will be crucial for scientists on Sage to work out what the rise in hospitalisations – and potentially deaths – might look like in the months ahead. Ferguson said: “One of the key things we want to resolve in the next few weeks is do we see an uptick in hospitalisations – we are seeing it in some areas – matching the cases, and what is the ratio between the two, because vaccination has substantially changed that.” Evidence is firming up around the Delta variant being 60% more transmissible than the Alpha variant, with estimates ranging from 40% and 80%. The variant is somewhat resistant to vaccines, particularly after one dose. While Ferguson believes we may see fewer deaths in the third wave compared with in January, the latest modelling does not rule out what he called a “disastrous” third wave if transmission and vaccine resistance are at the higher end of the best estimates. The latest official data showed 7,540 new confirmed cases of the virus in England. Hospitalisations are not yet rising sharply nationwide, though they are surging in hotspot areas including Greater Manchester. Chris Hopson, the chief executive of NHS Providers, said trusts in hard-hit areas were confirming that the vaccines provide good protection against the virus. “There is a growing sense that thanks to the vaccine, the chain seen in previous waves between rising infections and high rates of hospital admissions and deaths has been broken. That feels very significant,” he wrote in a blogpost for the British Medical Journal. But Hopson warned that the NHS was already “running hot” in many areas, and an increase in Covid admissions would set back efforts to tackle the long backlog of treatment for other health problems that has been caused by the crisis.