He urged the Government to avoid setting the mandatory deadline during winter months, “when the NHS is at its most stretched” or else it could risk a mass exodus of staff leaving the health service because they do not want the vaccine.
It comes after Health Secretary Sajid Javid said last week that he was "leaning towards" making jabs compulsory for healthcare staff in England, with around 100,000 NHS workers not fully vaccinated.
The Government could announce the change as early as this week, experts have said.
Mr Hopson told BBC Breakfast: “If we lose very large numbers of unvaccinated staff, particularly over the winter period, then that also constitutes a risk to patient safety and quality of care.”
He continued: “We’ve got a very, very difficult winter coming up and we know the NHS is going to be absolutely at full stretch.
“So it makes sense to set the deadline once that winter period has passed.
“We know that January, February, often early March is very busy, so that’s why we’re saying today that we think an April 2022 deadline is a sensible time.”
Mandatory jabs have already been introduced in the social care sector, unless medically exempt.
Addressing the issue in social care, he added: “In mid June, the Government announced it was going to be moving towards mandating staff vaccination, and the deadline they said was mid-November, so there was a five-month run-up.
“And what we’re saying in the NHS is we need that length of run-up as well.
“You just need to look at the problems that social care providers are currently reporting and saying: ‘look, we are really, really struggling at the moment in terms of staff potentially leaving just at the point when we need them’.
“And indeed some NHS staff are now having to help out, for example in Cornwall, are having to go and help out in the social care sector to ensure that we can discharge people from hospital.”
Two staff groups which appear to have lower vaccination take-up rates are women considering having children soon and NHS workers from black communities, he added.
He said that trust leaders have been having “supportive, encouraging” conversations with vaccine-hesitant staff to drive up vaccine take-up, adding: “One of the things we’re saying today is please can we ensure that we don’t have to quick a deadline so that we can carry on that process and, crucially, we can get through winter.”
Mr Hopson also tweeted that the “overall tone and approach must avoid scapegoating or denigrating those who are not currently vaccinated”.
He wrote: “Trust leaders are clear that progress up to now with vaccine hesitant staff has been based on encouragement and explanation, not coercion and control.”