The "trade offs" involved in lifting remaining COVID restrictions on 21 June must be subjected to "informed, evidence based" debate, a senior health leader has warned.
"It’s very clear that the vaccines are working," Chris Hopson told Sky News, while warning that some hospitals are "busy and pressured".
Any significant rise in COVID-19 patients may cause trusts to reduce non-urgent operations, the head of NHS Providers added.
And he said people are arriving at A&E departments with complex needs.
Medics suspect they have "held off from coming in but it’s now got to a point in their illness or condition that they really do have to have treatment", Mr Hopson explained.
His comments preceded an ambulance trust – South Western Ambulance Service – declaring a critical incident because of "extreme pressures" on its services.
Covering an area from Gloucestershire to Cornwall and across to Dorset and Wiltshire, it said some patients could wait longer for paramedics while others may be advised to seek alternative treatment.
Mr Hopson, who described the decision about whether to open up fully on 21 June as "difficult", is calling for a "better quality and richer debate than we have had on these issues in previous phases".
The Indian variant is not yet causing major pressure on hospitals, he said.
Admissions "vary between climbing slowly, plateauing or, interestingly, starting to reduce", he tweeted.
In one "key hotspot trust" there were 47 COVID inpatients on Saturday – down from 49 on Friday.
That is in "stark contrast" to peaks of 150 in January/February and 170 last November, he added.
Mr Hopson said the "significant majority of hospital cases are unvaccinated", with patients "significantly younger" than before, which "generally means less need for critical care".
He said no more than a "handful" of inpatients have had both jabs, and those who are admitted "usually have pronounced co-morbidities".
Community infection rates are levelling off, Mr Hopson said, with hospital leaders starting to believe they are unlikely to be overwhelmed as in previous peaks.
They are describing this as "clear, on the ground, confirmation of the success of the vaccination campaign", Mr Hopson added.
But one trust chief executive on the south coast told him that bed occupancy was already running at "96/97%".
Another trust boss, in a COVID hotspot, said they were trying to keep virus inpatients below 10 – a tiny fraction of the number in January and February.
As soon as the figure goes above that, wards in which patients are recovering from elective procedures will need to be converted to COVID wards, the CEO explained.
And trusts are reporting "significant increases" in the number of patients with severe mental health needs, "particularly children and younger people needing complex care", Mr Hopson said.
Sky News recently found almost 40% of children with mental health problems are waiting longer than NHS guidelines allow.
South Western Ambulance Service said it had responded to an average of 2,913 incidents a day last week, compared with 2,816 incidents a day between 12 April and 16 May.
On Saturday, it responded to nearly 3,200 incidents and it predicted similar levels across the weekend. It was the highest level of activity in 2021. The trust normally deals with around 2,650 emergency incidents a day.
Looking ahead to 21 June, Mr Hopson said hospital chiefs were concerned about the apparent increased transmissibility of the Indian variant.
Many people have still not had both jabs, and it takes three weeks for doses to fully take effect, he added.
He said the long-term impacts of COVID are "still largely unknown" and "trust leaders are clear that we shouldn’t just focus on mortality rates".
But Mr Hopson said the success of the vaccination campaign meant much lower levels of hospitalisation and serious illness – even with the new variant.
"This means there is a difficult decision to make for 21 June," he added.