A prioritisation list for 'phase one' of the rollout followed the announcement. Those who are under 50, don't have underlying conditions and aren't frontline health and social care workers await a 'phase two' list - but what about people who can't take the vaccine at all?
Deputy Chief Medical Officer Professor Jonathan Van-Tam said that pregnant women should hold off from getting the jab.
And the UK regulator has urged people with a history of “significant” allergic reactions not to take the jab after two people who had the vaccine on Tuesday had allergic reaction.
It's emerged that some people with weakened immune systems should not take the vaccine, while others might not respond to immunisation at all.
All in all, that's hundreds of thousands of people in the UK who should not have the vaccine.
Why have people with severe allergic reactions been told not to take the vaccine?
Two NHS staff members who received the jab on the first day of the mass vaccination programme suffered an allergic reaction, the NHS in England has confirmed. Both are recovering well.
The NHS in England said all trusts involved with the vaccination programme have been informed.
It is understood that both the staff members had a significant history of allergic reactions - to the extent where they need to carry an adrenaline auto injector with them.
They developed symptoms of "anaphylactoid reaction" shortly after receiving the vaccine and both have recovered after the appropriate treatment.
The Medicines and Healthcare products Regulatory Agency (MHRA) has given precautionary advice to NHS trusts that anyone who has a history of "significant" allergic reactions to medicines, food or vaccines should not receive the vaccine.
Professor Stephen Powis, national medical director for the NHS in England, said: "As is common with new vaccines the MHRA have advised on a precautionary basis that people with a significant history of allergic reactions do not receive this vaccination after two people with a history of significant allergic reactions responded adversely yesterday. Both are recovering well."
Why are pregnant women being advised against having the jab?
The Joint Committee on Vaccination and Immunisation (JVCI) has advised that pregnant women do not get the vaccine because there is “no data as yet on the safety of Covid-19 vaccines in pregnancy”.
The same guidance applies to women who think they may be pregnant or are planning a pregnancy within three months of the first dose.
Professor Jonathan Van-Tam added that none of the trials deliberately included pregnant women, but there were women who volunteered for the trial who then became pregnant.
He said the manufacturers will follow these women and their babies in their early years, saying: “But, right now, do we have the data in pregnant women to understand the use of any of these vaccines in pregnant women? No, we don’t."
He added that the guidance for pregnant women is a precaution and not a sign that the JCVI has identified a "terrible problem".
What about people with compromised immune systems?
Some people without a functioning immune system cannot be given the vaccine.
They may not have a working immune system because they take immunosuppressant drugs or have an immune deficiency.
Kate Bingham chairwoman of the UK’s Vaccine Taskforce explained that "vaccines work in people who have a functional immune system".
However, she added that there may be another option for those who are “immuno-suppressed and are going through bone marrow transplants, or indications or treatments that actually reduce [their] ability to mount an immune response".
What will be done to protect the immunocompromised from coronavirus?
The UK is to begin clinical trials of a new coronavirus antibody treatment aimed at people with a weakened immune system who cannot be vaccinated.
Developed by drugs giant AstraZeneca, the trial aims to evaluate the safety and effectiveness of a combination of two long-acting monoclonal antibodies – man-made proteins that act like natural human antibodies in the immune system.
The treatment can be injected or administered intravenously. A participant in Wakefield, West Yorkshire, will be the first in the world to receive it.
Sir Mene Pangalos, executive vice president of biopharmaceuticals R&D at AstraZeneca, said: “There is going to be a significant number of people – even in a world where vaccines are highly effective – who will not respond to vaccines, or in fact will not take vaccines.
“So having monoclonal antibodies as potential therapeutics is also important.”
The UK government has an in-principle agreement to secure access to one million doses of the antibody combination, dubbed AZD7442, if it is successful in the phase three trials.
Initial results from the randomised control trial are expected to be published in the first half of 2021, although the trial is expected to last for 12 months.
What will be done to protect pregnant women?
JCVI has said it anticipates data which will "inform discussions on vaccination in pregnancy". When this data becomes available, it will review its current guidance.
Although there's no evidence that pregnant women are more likely to get seriously ill from coronavirus, they are currently in the list of people at moderate risk as a precaution.
Therefore, the NHS advises they stay at home as much as possible, avoid anyone with coronavirus symptoms and follow social distancing advice.
Can pregnant women and immunocompromised people take other vaccines?
According to the World Health Organisation, live activated vaccines (LAVs) pose a "theoretical risk" to the foetus and pregnant women are generally advised not to take them as a precautionary measure. LAVs include jabs for measles and yellow fever.
On the other hand, WHO has said there is no evidence of adverse outcomes when pregnant women are given inactivated vaccines. Thus, it is generally safe for them to receive jabs for illnesses like the flu and tetanus.
If you have a weakened immune system, whether or not you can take a vaccine depends on a number of factors, like your medication and condition. If you are unsure about taking a vaccine, you should consult your doctor.
However, WHO recommends that severely immunocompromised people don't take the following vaccines:
DTP (diptheria, pertussis and tetanus)
It adds that with both pregnant and immunocompromised people, the potential risks of any vaccine need to be weighed against the benefits in those who may be particularly vulnerable to a disease.