Sir Simon Stevens explained that the Pfizer/BioNTech vaccine comes in packs of 975 people’s doses, which means it cannot be distributed to the thousands of care homes across England with smaller numbers of residents.
And the government is not permitted to split the packs into smaller deliveries until the regulator approves it.
The announcement risks disappointing elderly care home residents, identified as being clinically the top priority for a jab by official vaccination advisers, and their families.
Stevens told a Downing Street briefing: “If the MHRA, the independent regulator, give approval for a safe way of splitting these packs of 975 doses, then the good news that we will be able to start distributing those to care homes.”
Stevens said the rollout of the vaccine was “logistically complicated” because it has to be stored at -70C then moved around the country in a “carefully controlled way” with only a limited number of “further movements”.
It will therefore initially be rolled out to 50 “hospital hubs” around the country, which will start to offer jabs to over-80s, for example those with outpatient appointments next week, and care home staff.
Following this, GP practices will come together to operate 1,000 local vaccination centres across England.
When more vaccines are available, large vaccination centres and local community pharmacists will begin to offer jabs, “probably from the beginning of January”.
Because two jabs 21 days apart are required to deliver immunity, those who get the first dose in December will have a second one reserved, making it harder to vaccinate new people until the first wave is complete.
Stevens suggested the NHS was seeking to vaccinate all the most vulnerable groups by March or April, with the bulk of jabs being administered from January.
“In a nutshell this is going to be a phased programme,” he said. “We the NHS – your GP or your hospital – the NHS will contact you if you are ready to be offered the vaccination.
“Today of course is a day of great optimism but for all the reasons I’ve said I think it needs to be tempered with realism around phasing.”
Elderly people in care homes and care home workers are first on the priority list to receive the vaccine as determined by the Joint Committee on Vaccination and Immunisation (JCVI).
But ministers are facing questions about the delivery of the jab to care homes due to the very cold temperatures at which it needs storing.
The Pfizer/BioNTech vaccine has to be stored at -70C and is also stable at 2C to 8C for a short time, which has raised concerns about logistical challenges.
Downing Street earlier appeared to suggest that care home residents would start to get the vaccine next week.
Asked if they would get the vaccine within days, a No.10 spokesperson told reporters: “Yes, you have seen the prioritisation list with care home residents being right at the top of that list.”
But Johnson earlier warned of “logistical challenges to be overcome to get vulnerable people the access to the vaccine that they need”.
Health secretary Matt Hancock said that delivery of the vaccine would be according to the clinical priority list but also “operational necessity”, because the required cold temperature “makes this vaccine particularly challenging to deploy”.
The Welsh government went much further on Wednesday morning, saying it would not be able to deliver it to care homes at all for “practical” reasons and accepting some residents would miss out as a result.
“At this stage we cannot deliver this vaccine to care homes,” Welsh health minister Vaughan Gething said.
“That means we’re going to have a smaller number of vaccination centres that we’ll need to bring people to.
“Some care home residents therefore won’t be within the first few weeks of delivery of that vaccine.”
Pfizer and BioNTech have said the jab can be sent to care homes, as long as the vaccine travels for no more than six hours after it leaves cold storage and is then put in a normal fridge at 2C to 8C.
Ben Osborn, Pfizer’s UK country manager, told reporters: “The point I really want to emphasise is, at the point of administration and deployment by the NHS, our vaccine can be stored under normal refrigerated temperatures at 2C to 8C for five days.
“And that gives us the flexibility to reach the target populations identified this morning by the JCVI over the months ahead.”