The NHS has warned there will be a month-long "significant reduction" in weekly vaccine supply from the end of March.
The news of vaccine supply issues comes as the EU threatens to block the export of jabs to countries with high inoculation rates, such as the UK.
So what will this mean for people over 50 still waiting for their jab? And will this slow down the vaccine roll-out for everyone else?
Here's what we know so far:
What has the NHS said about the vaccine supply?
A letter from NHS England leaders to local health organisations said: "The government's Vaccines Taskforce have now notified us that there will be a significant reduction in weekly supply available from manufacturers beginning in the week commencing March 29, meaning volumes for first doses will be significantly constrained."
Health Secretary Matt Hancock said the reduction in available doses of the Oxford/AstraZeneca jab will likely begin on Monday, March 29, and last for around four weeks.
Mr Hancock said the issues were caused by a need to retest 1.7 million doses and a delay in deliveries from the Serum Institute of India (SII) where many of the vaccines are being produced.
At a briefing on the vaccine shortage on Thursday, Prime Minister Boris Johnson echoed Mr Hancock and added that the Indian government had not blocked exports of the jab.
I have booked an appointment, will it get cancelled?
In theory, no.
Vaccine centres and services have been told to close unfilled bookings from March 29 and to ensure that "no further appointments are uploaded to the National Booking System or Local Booking Systems from April 1 to 30."
The priority will be to focus on the first nine priority groups in phase one of the vaccine roll-out.
I'm over 50, can I still get the vaccine?
Mr Hancock has insisted vaccine appointment for the over-50s will still go ahead - they have been invited to book an appointment through the NHS website.
He also said that the April 15 target for vaccinating this age group, along with other phase one priority groups, with at least one vaccine dose would be met.
Phase one priority groups (to be offered first dose by mid-April)
1 - Residents in a care home for older adults and their carers (800,000 people)2 - Those aged 80 and over and frontline health and social care workers (a total of 7.1 million people in this group: 3.3m over 80s, 2.4m healthcare workers, 1.4m social care workers)3 - Those aged 75 and over (2.3 million)4 - Those aged 70 and over and clinically extremely vulnerable individuals (4.4 million)5 - Those aged 65 and over (2.9 million)6 - All individuals aged 16 to 64 with underlying health conditions which put them at higher risk of serious disease and mortality (7.3 million)7 - Those aged 60 and over (1.8 million)8 - Those aged 55 years and over (2.4 million)9 - Those aged 50 years of age and over (2.8 million)Phase two (to be offered first dose by mid-July)10 - All those aged 40-49 years11 - All those aged 30-39 years12 - All those aged 18-29 years
What about the under 50s?
Millions of people under 50 face the prospect of having to wait until after April 30 for their first vaccine appointment.
With the constraints on vaccine supply expected from late March, the health secretary said at a government Covid update on Wednesday that he wanted to ensure “every last vulnerable person” receives a jab before moving on to the under-50s group.
“We will do all we can and do everything necessary to ensure the supplies that are contractually committed to protecting people in this country,” Mr Hancock said.
The NHS letter said that inviting people for jabs who are not in the top nine priority groups is “only permissible in exceptional circumstances”.
It adds: “Those aged 49 years or younger should not be offered vaccination unless they are eligible via a higher cohort because they are e.g., clinically vulnerable, unpaid carer or frontline health and care workers.”
What about a possible block on doses from the European Union?
The UK’s success in its campaign has also contributed to tensions with Brussels as European Commission chief Ursula von der Leyen warned the bloc “will reflect on whether exports to countries who have higher vaccination rates than us are still proportionate”.
President von der Leyen said she wanted “reciprocity and proportionality” in exports, pointing out that 10 million doses of vaccine had gone from the EU to the UK.
Although Pfizer jabs were crossing the English Channel to the UK, AstraZeneca vaccines are not heading the other way, she indicated.
“We are still waiting for doses to come from the UK, so this is an invitation to show us that there are also doses from the UK coming to the European Union so that we have reciprocity,” she added.
Speaking on Wednesday evening, Mr Hancock said the supply of vaccines to the UK from EU production facilities was “fulfilling contractual responsibilities and we fully expect those contracts to be delivered on”.
He said the UK had funded research into the Oxford/AstraZeneca vaccine and had a contract for the first 100 million doses for people in the UK.
ITV News Editor James Mates said that if the EU was to block doses of the vaccine, it could have a big impact on the vaccine roll-out but European leaders may not support a move.
What are the vaccine companies saying?
A Pfizer spokesperson said deliveries “remain on track” for the first quarter of its 40 million dose agreement with the UK.
She added: “We will work closely with the government to ensure this remains the case; our overall projected supply for Q2 remains unchanged and we are on course to continue to deliver a steady supply of vaccines to the UK, April through to June, in line with our contractual commitments.
“Based on current projections, we believe that we can deliver more than two billion doses of the Pfizer/BioNTech vaccine worldwide by the end of 2021 – an increase from the 1.3 billion doses initially projected. As of March 11, we have shipped 160 million doses of our vaccine worldwide and we are tripling the number of doses we deliver to the EU in Q2 compared to Q1.”
AstraZeneca has previously said that it is confident that its commitments are on track.
The UK has also secured 17 million doses of the Moderna vaccine and the government said they are still expecting supplies to arrive in the Spring.
What has been the NHS response?
The NHS Confederation, the independent membership body for the NHS, said it was reassured over the government’s commitment to the over 50s but concerned over blocking first doses for those outside the priority groups.
Ruth Rankine, director of the Confederation’s primary care network, said: “Putting an embargo on new first dose bookings for a whole month due to supply constraints beyond the NHS’s control will make this an even taller order.
“This is particularly true given that the events in Europe have knocked some patients’ confidence in confirming or attending their scheduled appointments.”
How has the roll-out gone so far?
The Department of Health and Social Care (DHSC) said that more than 25 million people have received their first vaccine and 1.7 million have had their second vaccine.
Officials said that 95% of people aged 65 and over have had their first dose, and nine in 10 of those clinically extremely vulnerable have received a first jab.
The figures, which come on the 100th day of the vaccination programme, show that overall, health services across the UK vaccinated 25,273,226 people between December 8 and March 16 with first doses of the Pfizer/BioNTech and Oxford/AstraZeneca vaccines.