Covid rules have been strengthened in response to concern over the newly identified Omicron variant.
The booster programme is also being expanded to millions more adults.
What are the new measures?
- Face coverings are once again compulsory in places like shops and on public transport in England
- Pupils at secondary schools in England are being "strongly advised" to wear face coverings in communal areas. The guidance also applies to staff and visitors at all schools and childcare settings
- Secondary school pupils in Wales are being urged to wear face masks in classrooms and communal areas. Staff and students in colleges and universities should also wear face coverings inside
- All contacts of suspected Omicron cases will be contacted by their local contact tracing system and required to self-isolate for 10 days, regardless of their age or vaccination status
- Anyone entering the UK requires a PCR test within 48 hours of arrival and must self-isolate until they have a negative result
What's happening with boosters?
The booster campaign is being stepped up:
- Booster jabs will be offered to all over-18s
- The gap between second dose and booster will be reduced to three months from six
- People with weakened immune systems will be offered a fourth dose - as a booster - no sooner than three months after their third
- Children aged 12 to 15 will be invited for a second dose, three months after their first
Vulnerable people will be contacted first, followed by older age groups, then younger ones.
People are being urged not to try to book their booster until the NHS says it's their turn.
Every eligible person in England and Scotland should be able to book their booster by the end of January - although some may be given at a later date.
Wales will increase capacity in clinics and drive-through centres and will work with GPs and others - including soldiers and firefighters - to deliver vaccines.
Northern Ireland has announced extra vaccination clinics.
Scotland says it will set out its approach "very soon".
Why are the new changes being made?
A new Covid variant - Omicron - has been identified, and there are fears that it could be more infectious and less responsive to vaccines.
Precautions are being taken until scientists can properly assess what the new variant's impact could be.
What are the chances of another lockdown?
The government has repeatedly said it has no plans for another lockdown in England.
In September, Health Secretary Sajid Javid announced a list of measures which would come into place if the NHS came under "unsustainable pressure".
These included face coverings, but also measures which have not been brought in such as advising people to work from home and mandatory Covid passports.
What other restrictions are currently in place in England?
- 1m-plus social distancing remains in places such as hospitals and passport control
- people are advised to test for Covid before going to crowded places or spending time with those at greater risk
- Anyone testing positive has to self-isolate
What's the situation in Wales?
Current measures already included:
- NHS Covid Pass needed for entry to nightclubs, cinemas, theatres and concert halls and many other indoor and outdoor events
- working from home wherever possible
- face coverings compulsory on public transport, and in shops and hospitals
What's the situation in Northern Ireland?
- a limit of 30 people from different households mixing indoors in domestic settings
- face coverings compulsory in shops, indoor seated venues and visitor attractions, public transport and some other settings
- post-primary pupils must wear a face covering inside school buildings, as well as on school transport. Staff are encouraged to wear masks in school areas where they can't socially distance
- vaccine passports for venues including nightclubs, bars and cinemas
What restrictions are currently in place in Scotland?
- physical distancing rules remain in healthcare settings such as hospitals, GP surgeries and dentists, where the 2m (6ft) rule applies
- face coverings still compulsory on public transport and inside places like shops
- all school staff and secondary pupils to wear face coverings indoors
- indoor hospitality venues must collect customer contact details
- working from home where possible
- all over-18s required to prove their vaccine status at nightclubs and other venues