Here is a look at what the current situation is in the UK and what is expected to happen over the coming days and weeks.
What was the main announcement during Mr Johnson’s pre-recorded address to the nation on Sunday night?
The PM has brought forward the target of offering a Covid-19 booster jab to every adult by a month.
The target had previously been the end of January, but that has been brought forward to the end of December.
He said everyone eligible aged 18 and over in England will have the chance to get their booster before the new year.
How will this be achieved?
Mr Johnson said it will require an “extraordinary effort”.
He said the “emergency operation” will be assisted by deploying 42 military planning teams across every region and setting up additional vaccine sites and mobile units.
He said opening hours will be extended so that clinics are open seven days a week, with more appointments early in the morning, in the evening, and at weekends.
He also said thousands more volunteer vaccinators will be trained.
Will the efforts to give booster jabs to the nation affect other parts of the health service?
Yes. Mr Johnson said the focus on boosters and making the new target achievable will mean some other appointments will need to be postponed until the new year.
But he said that if this is not done now, then the wave of Omicron “could be so big that cancellations and disruptions, like the loss of cancer appointments, would be even greater next year”.
What did the PM say about the variant and what is currently known about it?
Mr Johnson said there is evidence that Omicron is doubling in the UK every two to three days, adding that we know from “bitter experience” how exponential curves develop.
He said there is “no doubt” that the UK faces a “tidal wave” of Omicron infections.
He said it was “clear” that two doses of a vaccine are “simply not enough to give the level of protection we all need”, but that a booster can bring the level of protection back up.
Mr Johnson said there are currently patients with Omicron in hospital, adding that at this point scientists cannot say the new variant is less severe than previous variants.
“And even if that proved to be true, we already know it is so much more transmissible that a wave of Omicron through a population that was not boosted would risk a level of hospitalisation that could overwhelm our NHS and lead sadly to very many deaths,” he said.
What is known about how effective two jabs are in comparison to two jabs plus a booster?
Analysis by the UK Health Security Agency (UKHSA) has found that the AstraZeneca and Pfizer vaccines provide “much lower” levels of protection against symptomatic infection with Omicron compared with Delta.
But the UKHSA said a booster dose gives around 70% to 75% protection against symptomatic infection with Omicron.
How many people in the UK have Omicron?
The UK, as of Sunday, recorded a further 1,239 confirmed cases of the Omicron mutation, bringing the total number of cases to 3,137 – a 65% increase from Saturday’s total of 1,898.
Education Secretary Nadhim Zahawi said the actual number was likely to be 10 times as high.
What is the UK Covid alert level currently set at?
On Sunday, just hours before the Prime Minister’s address to the nation, the UK Covid alert level was raised to Level 4, up from Level 3, following the rapid increase in the number of Omicron cases being recorded.
The recommendation was made to ministers by the country’s four chief medical officers and NHS England’s national medical director following advice from the UKHSA.
Are there projections for how many hospital admissions Omicron may cause in the UK?
Modelling made public on Saturday suggests that, under one scenario, almost twice the number of coronavirus patients could be admitted to hospital compared with last year due to the impact of Omicron.
Experts from the London School of Hygiene & Tropical Medicine (LSHTM) calculated figures which say a large wave of infections could occur over the next few months if tougher measures are not brought in.