Reporter Chloe Keedy explains what the new rules are and sets out the concerns of Boris Johnson and some of the UK's top scientists
A host of new measures have been introduced in a bid to "slow down" the spread of the Omicron Covid-19 variant after two cases were detected in the UK.
In England, face-coverings will again become mandatory in some settings, tougher testing rules will come into force for anyone who arrives in the country from abroad, contacts of people who have tested positive for the Omicron variant must self-isolate for 10 days, and booster vaccines will be rolled out to more people.
It comes after the health secretary announced two cases of the new variant had been detected in the UK - one in Brentwood, Essex, and another in Nottingham.
Sajid Javid said the individuals and all members of their households were told to enter self-isolation.
The UK Health Security Agency confirmed the cases, which are both believed to be connected and linked to travel to southern Africa, after genomic sequencing overnight.
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Leading a press conference alongside Chief Scientific Advisor Sir Patrick Vallance and Chief Medical Officer Professor Chris Whitty, the prime minister announced a host of new measures.Speaking from Downing Street, Boris Johnson said that 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, Boris Johnson has said in a press conference after it was announced that two cases of the Omicron Covid-19 variant have been detected in the UK.
Earlier on Saturday, Health Secretary Sajid Javid said 10 southern African countries are now on the travel red list, meaning UK and Irish residents arriving in the UK from any of them must quarantine in a hotel for 10 days. 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.
The prime minister also revealed in the update that 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. These people will be contacted by NHS Test and Trace.
He added that face coverings on public transport and in shops will return from next week, but will not be mandatory in hospitality venues.
The booster vaccine rollout will also be stepped up to cover more people and shorten the gap between second and third doses.
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.
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Mr Johnson said the new measures will be assessed in three weeks' time, adding: "At that point we should have much greater information about the continuing effectiveness of our vaccines."
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.
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.
Watch the press conference in full:
Mr Johnson 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”.
He said the extra measures were being brought in in a bid to "slow down the seeding" of the variant.
Prof Whitty said he expects Omicron numbers to continue to rise around the world as it spreads rapidly.
He continued that the new variant's "extensive mutations" mean “there is a reasonable chance there is some degree of vaccine escape”, but 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 chief medical officer added that this means there is an even greater need for everyone to have double vaccinations and boosters.
Sir Patrick agreed that the UK's most important defence against Omicron was currently vaccination.
Sir Patrick added that the current vaccines can in theory be easily tweaked for new variants, which could be achieved in 100 days, and that anti-viral drugs are also coming on stream.
Giving an update on the wider Covid situation in the UK, as a further 39,567 cases were recorded in the 24 hours to 9am on Saturday, Prof Whitty said the majority of cases in the UK remain the Delta variant.
He said there is currently significant rates of transmission among young people but rates among people aged over 60 and vulnerable groups are improving, meaning hospitalisations and deaths continue to decrease.