Boris Johnson said the "great national fightback" has begun against Omicron - almost three weeks after the first cases of the new variant were found in the UK.
He said it's "absolutely vital" for Brits to get their booster jabs as the Government is "throwing everything" at the strain.
The Prime Minister even launched a plea for Brits to volunteer and "help the nation get boosted now".
Professor Chris Whitty warned "records will be broken a lot over the next few week" as Omicron continues to grow at extremely fast rates.
But what about social gatherings? Here's what we learned...
1) England is facing TWO epidemics
In the latest press conference Professor Whitty warned the new wave of Covid is still more dangerous than the Delta variant, even if the new variant ends up being milder.
Chris Whitty said scientists are still gathering data on the Omicron variant, but added "all the things we do know are bad".
He expressed his concerns that records over the next few weeks may be "broken up" because of how fast Covid rates are spiralling.
"What we've got is two epidemics on top of one another - an existing Delta epidemic, roughly flat, and a very rapidly-growing Omicron epidemic on top of it."
He added: "Between the time that it first starts to really take off in a way people will be able to see and the point when we get to very, very large numbers will be quite a short one, and that, I think is part of the issue in trying to deal with this."
Omicron is already a dominant variant in London and is set to be larger than the Delta variant across the country within the next week.
2) The Omicron variant is far from 'mild'
The UK is facing a "really serious threat" from the Omicron variant, as it shows no signs of slowing down, England's Chief Medical Officer warned.
Today, the UK recorded its highest number of daily Covid cases since the start of the pandemic, with 78,610 new infections.
Professor Whitty said he expects Covid cases to go up further in the coming weeks and suggested the data from South Africa that shows the new variant is milder, is wrong.
He told the press conference: "There's a danger that people have over-interpreted this, think it's not going to be a problem.
"I want to be clear, this is going to be a problem".
3) Christmas parties can go ahead but Brits should be selective
Brits are still free to enjoy festive gatherings and parties in the run up to Christmas.
"I said many times that I thought this Christmas will be considerably better than last Christmas, and I stick to that," the Prime Minister said.
But Prof Whitty urged Brits to prioritise who they see, so they can reduce their social contacts.
Speaking alongside the PM, Prof Whitty said: "Don't mix with people you don't have to.
"[You] don't need a medical degree to realise that is a sensible thing to do with an incredibly infectious virus."
Dr Nikki Kanani apologised to her own kids live on TV for their scaled back plans in the run up to Christmas.
She said alongside Prof Whitty and the PM: "As for my Christmas plans, they're already scaling back. There will be far fewer things that I'll do with my children as we run-up to Christmas - and if they're listening, sorry, but that is absolutely a fact."
Prof Whitty encouraged people to take lateral flow tests before visiting vulnerable people.
And he said Brits should meet outdoors if possible, or in areas with good ventilation.
4) 12-15 year olds can get a second jab
From Monday December 20, the youngsters will be able to book appointments for their second Covid jab.
The Government have made it clear they want to keep kids in schools.
Mr Johnson said: "Let's all make sure our children and young people are vaccinated before they go back next term".
Earlier, Tory MPs had called on the Government to ensure schools remain open in the New Year to avoid safeguarding hazards and mental health problems.
A few weeks ago, Labour slammed the Government for the slow progress of their vaccine rollout.
They estimated some 12-15-year-olds could left waiting for their first jabs right into next year, despite being eligible from October half term.
Scientists have cut the need for Brits to wait to have their booster dose from six months to three, and this includes youngsters.
When the booster programme began, England's four Chief Medical Officers urged Brits not to stigmatise parents and people who refused the booster dose.
But in a complete turnaround, given the rapid transmission of Omicron, the PM said jabs will be available for Brits "wherever they are".
This confirmation comes after the UK's four chief medical officers recommended a universal rollout of the Pfizer jab to three million more children after concluding it would reduce outbreaks in schools.
They warned of the mental health impacts, particularly on children from deprived areas, of missing more classes this autumn and winter.
5) Large numbers of people will be hospitalised with Covid after Christmas
Prof Whitty said it is a "nailed on prospect" that hospitalisations will soar after the festive holidays because it is spreading at an "phenomenal pace".
He told the Downing Street press conference: "I'm afraid there will be an increasing number of Omicron patients going into the NHS, going into hospital and going into intensive care.
"Th exact ratios we don't yet know, but there will be substantial numbers and that will begin to become apparent, in my view, fairly soon after Christmas."
Earlier this week, the UK Health Security Agency (UKHSA) released an explosive estimate that daily infections were running at 200,000.
UKHSA boss Dr Jenny Harries warned MPs of a "staggering" number of cases over the next few days - which could plunge the NHS into "serious peril".
6) The Government has not ruled out further restrictions
The Prime Minister is doing his best to propel his vaccination campaign, so he can avoid introducing further restrictions libertarian Tories will despise.
He opened his press conference urging every eligible adult to book their jab and "get boosted now" as it's "absolutely vital that you do".
"We're going with Plan B as we said we would in the event of the numbers starting to rise and in the event of a new variant appearing. And that's that's the right thing to do," the PM told the press conference.
Mr Johnson urged Brits to get tested when going to venues like nightclubs and isolate if in contact with someone who has Omicron.
He said the advantages of having testing capacities, which we didn't have last year, coupled with the booster vaccination programme is enough at the moment.
Prof Whitty urged Brits to use "serious caution" when deciding which social gatherings to attend as hospitalisations were seen in South African cases of Omicron.
Earlier, Health Secretary Sajid Javid did not rule out introducing further restrictions over the coming weeks.
Speaking at the Gordon Hospital in Westminster, Mr Javid said case numbers are still rising "rapidly" but we are "buying time to assess this virus" with the increased roll out of Covid jabs.
The PM said their current measures are the right ones needed for now.
7) Boris Johnson insists he always follows Covid rules
When asked about Downing Street Christmas parties reported by The Mirror, the PM insisted he always follows the rules.
The PM appeared to distance himself from mounting accusations of lockdown breaching parties that took place in Westminster last Christmas, as he noted that politicians "should" follow Covid rules.
Mr Johnson said: "On your point about rules, I follow the rules. Everybody across politics should follow the rules."
It comes after Shaun Bailey quit his role in the London Assembly last night, and appeared to apologise on Twitter.
Prof Whitty said "it's a matter of fact" that people will get angry when people in "high places" break the rules.
He said: "When people think there is unfairness in high places of course they get angry. Most people separate that out from their absolute need to think about what is good for them, their families, their communities and their workplaces."
The bumbling PM retorted: "On your point about rules, I follow the rules. Everybody across politics should follow the rules."
