The New Year booster target has been met across the UK, according to government ministers.
The goal of offering all eligible adults a booster by 31 December was in response to the Omicron variant. Other measures include approval for a low-dose vaccine to be offered to vulnerable primary school children.
When can I have a booster?
In England boosters are available to over-18s - three months after the second dose. (Boosters can be booked after two months).
Some walk-in appointments may be available - depending on where you live.
In Scotland boosters can be booked online.
In Wales people should wait to be invited, with older and higher-risk people being prioritised.
In Northern Ireland people aged 18 to 29 can go to walk-in hubs, and make booster appointments.
Why do I need a booster?
Early studies from UK researchers suggest a booster vaccine - on top of the first two jabs - will provide 80-85% protection against Omicron (compared with 97% against Delta).
More antibodies are developed thanks to the booster, giving the body stronger defences against the virus.
It makes it harder for Omicron to infect the body, although current vaccines are still not a perfect match.
What vaccine will I get for my booster?
Your booster will be a single dose of either Pfizer or Moderna - regardless of which vaccine you received before.
If you tested positive for Covid recently, you should wait four weeks (28 days) from the date of the test before having your booster.
You shouldn't have the booster if you have a severe illness or high fever, but Pfizer and Moderna say a mild fever or a cold are not reasons to delay.
The vaccines do not give you an infection and will not cause positive results on a lateral flow or PCR test.
Which children are being vaccinated?
A low-dose version of the Pfizer vaccine has been approved for five to 11-year-olds with health conditions that put them at greater risk from catching Covid.
Primary school children who live with clinically vulnerable adults should also be offered a jab, government vaccine advisers said.
It is not yet clear when children would get the doses.
The Joint Committee on Vaccination and Immunisation also recommended the normal booster dose should be offered to children aged:
* 16 and 17
* 12 to 15 if they are in an at-risk group or live with someone who is immunosuppressed
* 12 to 15 who have a severely weakened immune system, who should get four doses
All children aged 12 and over are being offered two doses of the Pfizer jab. They can usually have a second dose 12 weeks after the first.
Is the Covid vaccine compulsory?
Not in the UK, although a number of European countries have tightened restrictions for the unvaccinated.
However, all front-line NHS staff in England (with some exceptions) must be fully vaccinated by 1 April.
Care home staff in England must already be vaccinated (unless exempt).
Across the UK, Covid passports are required to enter some venues.
What if I haven't had my first or second vaccine?
You can still book your first or second jab. You need to wait eight weeks between the first and second.
In particular, the government wants unvaccinated pregnant women to come forward.
* In England, bookings for over-12s can be made online, by calling 119, or visiting a walk-in clinic
* In Scotland, over-16s can register for their first or second vaccine online, or by calling 0800 030 8013
* In Wales, adults can contact their local health board
* In Northern Ireland, you can book online or call 0300 200 7813; walk-in centres are open to older teenagers
What about side effects?
The most common ones include a sore arm, headache, chills, fatigue and nausea.
They are part of the body's normal immune response to vaccines and tend to resolve within a day or two.
There are extremely rare, but occasionally fatal, cases of people developing blood clots after taking the AstraZeneca vaccine.
And a very small number of people have experienced a severe allergic reaction after the Pfizer vaccine.
You should discuss any existing serious allergies with your healthcare professional before being vaccinated.