The number of people in hospital with the Covid variant now known as Delta is rising but not "very significantly," the boss of NHS Providers has said.
Chris Hopson said many of those in hospital in Bolton - the area worst hit by the variant - were younger, with "very few" fully vaccinated patients.
He said this appeared to show vaccines had "broken the chain" between infection and serious illness.
But a scientist pointed to "concerning signs" of the variant's spread.
Mr Hopson was speaking to BBC Breakfast a day after the latest national coronavirus statistics showed that there had been 6,238 new positive cases, and that daily rate has doubled in just three days.
That surge is being driven by the increased transmissibility of the Delta variant, which was first identified in India.
But Mr Hopson said that in Bolton, people in hospital with Covid were "a lot younger" than patients in earlier stages of the pandemic, which meant there was "less demand on critical care".
He added that there were "very, very few" people in hospital who had had both doses of a Covid vaccine, as they had the "build-up of protection after those jabs".
Mr Hopson said in the latest phase of pandemic, the number of people in hospital in Bolton with Covid had not gone past 50, compared with a peak of 170 in November.
He added: "Infection rates have been increasing in a number of different places. We know that the hospitalisations are increasing, the rates of people coming into hospital in those areas are rising. But they are not rising very significantly."
But he warned that the backlog of care for non-Covid illnesses caused by the lockdown means that even a small rise in the number of Covid patients could see hospitals become overstretched.
Asked if the lifting of all restrictions on social contact on 21 June should go ahead as planned, Mr Hopson said the government's decision has become "significantly more finely balanced" due to the NHS still being busy, the Delta variant's increased transmissibility and because many people are still unvaccinated.
He said that he now suspects that, instead of 21 June being an "all or nothing" day when remaining lockdown restrictions are lifted, there may be some things that will be able to change from that day - but it will be "too high risk" to relax other measures.
Public Health England has said that the
Delta variant is now the dominant strain of coronavirus in the UK, with the number of cases confirmed by laboratory analysis rising by 79% over the last week to 12,431.
The most affected area of the country remains north-west England, PHE said, with confirmed cases in Bolton rising by 795 over the past week to 2,149. Blackburn with Darwen has seen a near doubling of cases to 724 in total.