A host of new measures have been introduced in a bid to "slow down" the spread of the Omicron Covid-19 variant after three cases were detected in the UK.
Hours after Health Secretary Sajid Javid announced that two infections were identified in Nottingham and Brentwood, Essex, and before a third case was confirmed, Boris Johnson gave a press conference alongside the Chief Scientific Advisor Sir Patrick Vallance and Chief Medical Officer Professor Chris Whitty.
Prof Whitty warned that the new variant's "extensive mutations" mean “there is a reasonable chance there is some degree of vaccine escape”, but said he is hopeful that current jabs can still prevent severe disease even if the vaccine does not prevent it spreading as much as would be desirable.
The prime minister said that the current scientific understanding is that the Omicron variant “spreads very rapidly and can be spread between people who have been double-vaccinated”.
While the trio warned that cases of the new variant would likely rise, the PM said the extra measures were being brought in in a bid to "slow down the seeding" of the variant.
So, what has been announced and what does it mean for Christmas?
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Anyone who travels abroad must take a PCR test on day two of their arrival back in the UK and self-isolate until they get a negative result. The new measure applies to passengers arriving into the UK from 4am on Tuesday.
Earlier on Saturday, Mr Javid announced that Angola, Mozambique, Malawi and Zambia will join South Africa, Botswana, Eswatini, Zimbabwe, Namibia and Lesotho on the travel red list.
From 4am on Sunday, UK and Irish residents who arrive back into the country from one of these countries must quarantine in a government-approved hotel for 10 days at a cost of £2,285 for one adult. These packages can be booked through the government's website.
The red list rules apply to people of all ages and vaccination status and anyone who breaks the rules faces a fine of up to £10,000.
Non-UK and Irish residents who have been in the affected countries in the previous 10 days will be refused entry into England from midday on Friday, November 26.
Anyone who comes into contact with someone who has tested positive for the Omicron variant must isolate at home for 10 days, regardless of their vaccination status.
If you need to quarantine you will be contacted by NHS Test and Trace.
After a third case of the Omicron variant was confirmed on Sunday, the government announced that all staff, visitors and pupils in Year 7 – the first year of secondary school – or above, are “strongly advised” to wear a covering in communal areas in England's schools and colleges, unless they are exempt.
The temporary guidance will apply from Monday and covers all education establishments including universities, as well as childcare settings such as early years care.Face coverings on public transport and in shops in England will return from Tuesday November 30, but will not be mandatory in hospitality venues.
The booster vaccine rollout will be stepped up to cover more people and the gap between second and third doses will be shortened.
Prof Whitty said the Joint Committee on Vaccination and Immunisation will now need to decide whether to extend the booster vaccine down to adults age 18, and whether a second dose should be offered to children aged 12-15 who decided with their families to get the first dose of the vaccine.
Currently, adults aged 40 and over, those with underlying health conditions, and frontline health and social care workers are all eligible for a third or booster jab six months after their first.
Children aged between 12 and 15 are currently only being offered one dose of a coronavirus vaccine.
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Do these rules apply to all of the UK?
Health is a devolved matter so any announcements made by Mr Johnson only apply to England.
Mask-wearing is already mandatory indoors and on public transport in Wales, Scotland and Northern Ireland, and some form of vaccine passport or negative Covid test is required to get access to certain indoor events in these countries.
The Welsh and Scottish governments have said that their rules will mirror those set out by the prime minister. Northern Ireland is expected to follow suite.
What does this all mean for Christmas?
The new measures will be assessed in three weeks' time when "we should have much greater information about the continuing effectiveness of our vaccines", Mr Johnson said.
However, Sir Patrick warned that the UK may need to "face up" to the possibility of further action if the Omicron variant is very transmissible.
When questioned if it meant that Christmas plans could be curtailed for a second year running, Mr Johnson said he is “absolutely confident that this Christmas will be better than last Christmas”, suggesting he has no current plans to introduce a lockdown.
He said the country is in a “strong position” ahead of the festive period but the “best thing to do” is to keep being jabbed.
Face coverings will not be mandatory in hospitality settings, meaning Christmas parties in pubs and restaurants are able to go ahead as normal.
On Sunday, Mr Javid has said people should plan for Christmas as “normal”.
He told Sky’s Trevor Phillips on Sunday: “I think it’s fair to say that the nature of this pandemic is it would be irresponsible to make guarantees.
“As for Christmas, I think people should continue with their plans as normal for Christmas, I think it’s going to be a great Christmas.”