"Without a doubt, the ones who are getting very sick are the unvaccinated."

Dr John Thomson, an emergency consultant at Aberdeen Royal Infirmary, is among those on the frontline of what is shaping up to be the toughest January ever seen in the NHS.

An explosion in Covid cases driven by the Omicron variant is starting to filter through to hospitals, with the number of patients with the virus soaring from 524 on Christmas Eve to more than 1,300 now.

Boosters appear to provide strong protection, cutting the risk of hospitalisation by around 88 per cent.

HeraldScotland: The number of Covid admissions has risen from an average of 64 per day in mid-December to 134 per day by January 3. The peak last January was 200 per dayThe number of Covid admissions has risen from an average of 64 per day in mid-December to 134 per day by January 3. The peak last January was 200 per day

However, given that this compares to 95% with the Delta strain, that means that twice as many 'boosted' individuals will be susceptible to serious illness this winter than if Omicron had never emerged.

Meanwhile, its sheer transmissibility means that it is more likely than ever to find and infect the 300,000 adults in Scotland who remain unvaccinated.

According to Public Health Scotland (PHS) figures for mid-December, rates of hospitalisation per 100,000 were 12 times higher among the unvaccinated than the triple-jabbed, even after adjusting for differences in age between the groups.

READ MORE: Six in 10 Covid patients admitted 'because of' infections

"I've personally dealt with patients in the last month, both of whom subsequently died, and they were both less than 40 and both unvaccinated," said Thomson, who is vice-president for Scotland at the Royal College of Emergency Medicine.

"They were both overweight, which we know is a risk factor, but they didn't have any other significant medical history.

"They were what we'd call 'young and fit' because they don't have chronic medical problems and had been entirely well, until they became unwell with Covid."

HeraldScotland: Dr John Thomson (inset) said the sickest Covid patients turning up at A&E are unvaccinatedDr John Thomson (inset) said the sickest Covid patients turning up at A&E are unvaccinated

In intensive care, roughly 50-60% of Covid patients are unvaccinated despite accounting for less than 10% of the general adult population, with a significant portion of the rest being immuno-suppressed patients in whom the vaccine has failed.

So far ICU Covid numbers have been relatively flat and appear lower than in previous waves.

By Friday, there were 48 ICU patients out of a total of 1,323 in hospital; at the peak of the Delta wave in September, when there were 1,107 people in hospital with Covid, 94 were in ICU.

However, of the 193,000 suspected Omicron cases so far in Scotland, only 18,000 have been detected in the over-60s.

READ MORE: Testing and isolating has to adapt to a post-vaccine pandemic

Dr Naz Lone, an intensive care medic and chair of the Scottish Intensive Care Society Audit Group (SICSAG) - which has been tracking the impact of the pandemic on ICUs - said it is too early to be sure that Omicron won't cause a spike in critical illness.

He said: "We're watching and waiting - I don't think anyone is thinking 'great, this is a mild disease'.

"We're waiting to see, as the virus ripples up into older age groups and the effect of intergenerational mixing at Christmas comes through, how well vaccines are holding up.

"Especially in people who are double vaccinated but not boosted, because their immunity will be waning.

"It might be a few weeks before we can say with confidence that we're not going to see major changes in ICU numbers of Covid patients."

HeraldScotland: Daily ICU Covid admissions have been rising since December 31Daily ICU Covid admissions have been rising since December 31

Much of the focus recently has been on differentiating Covid patients in hospital "because of" infections from those who happen to test positive after being admitted for something else.

Preliminary analysis published on Friday suggests the former has fallen to 60%, compared to 68% between March and August, but there are several caveats.

Firstly, it was based on a small sample (just 126 patients) from two health boards over six days - December 30 to January 4 - when admissions were still a combination of Delta and Omicron (the Omicron variant only overtook Delta in mid-December).

Broken down, 26 had the Delta variant, 14 were Omicron, and in 86 cases it was unknown which strain had caused the patients' infections.

Curiously though, when PHS did compare known Omicron and Delta patients, they found that 79% of Omicron patients were in hospital 'because of' Covid compared to 46% of those infected with Delta - though they stressed that the numbers involved are "too small to draw any substantial conclusion" about Omicron's virulence.

HeraldScotland: NHS absences due to Covid infections have more than doubled in just three weeksNHS absences due to Covid infections have more than doubled in just three weeks

For those on the frontline, however, the question of whether or not Omicron causes milder disease is almost irrelevant; it is the number of people becoming infected which is causing major disruption.

The number of NHS staff absent after testing positive for Covid has spiralled from 2,300 to nearly 5,500 in just three weeks.

Army personnel are now being deployed to hospitals in Lanarkshire, Ayrshire and Grampian to support struggling A&E and intensive care departments.

Even before Omicron, shortages of ICU nurses in particular were making it impossible to safely staff the 173 ventilated beds NHS Scotland is supposed to have as a baseline.

HeraldScotland: Intensive care admissions for Covid were falling steadily in Scotland until the end of January, but that decline stopped suddenly three weeks ago and it is unclear exactly why

Record Covid absences in social care is also having a knock-on effect for hospitals, with more than 1,500 beds currently occupied by patients well enough to leave but unable to be discharged.

A combination of beds increasingly 'blocked' by elderly patients and staff shortages meaning that fewer hospital beds can be safely provided is causing bottlenecks which leave patients stranded in ambulances or stuck for hours in A&E.

"On New Year's Day, we had two patients wait beyond 21 hours - one waiting for an ambulance to take them back to their care home and one waiting for a bed," said Thomson.

"Then, later that day we had 10 ambulances stacking [outside A&E] which is probably, for my own department, if not the highest number ever, very close to it."

READ MORE: Why the NHS winter crisis is about staffing, not Covid 

The decision to cut isolation for the Covid positive from 10 to seven days should enable a majority to return to work three days earlier - but around 15% will still be infectious by day seven and required to complete a full 10-day quarantine.

With current pandemic modelling suggesting that infections in Scotland will not peak until mid-January - when they hit 30,000 to 90,000 per day - the full impact of the Omicron wave is yet to be felt.

HeraldScotland: The modelling by Scottish Government epidemiologists combines confirmed cases with estimates for the numbers of undetected and asymptomatic cases in the community to forecast potential trajectories based on factors such as behaviour and vaccine efficacyThe modelling by Scottish Government epidemiologists combines confirmed cases with estimates for the numbers of undetected and asymptomatic cases in the community to forecast potential trajectories based on factors such as behaviour and vaccine efficacy

Also misunderstood is just how problematic 'incidental' Covid is for hospitals.

"It's very challenging," said Dr Antonia Ho, an infectious disease specialist who works at Monklands and teaches at Glasgow University.

"Just because they don't need care for their Covid, for infection control reasons they still need to be isolated. Hospitals keep having to change green wards to red wards regardless of whether patients are symptomatic or not.

"Then, with more and more Covid in hospitals - incidental or symptomatic - you're increasing the risk of nosocomial transmission."

The latest data for the week ending December 12 - pre-dating the Covid hospital surge - shows that 151 patients tested positive from swabs taken after admission; 51 had almost certainly caught the virus while in hospital.

HeraldScotland: The cancellation of routine orthopaedics was announced tonight

While life-saving, emergency procedures will continue even if a patient tests positive for Covid, non-urgent tests, treatment and operations are stalled by a strict 14-day isolation period regardless of patients' vaccination status.

"Whether they're symptomatic or not, it impacts on care," said Ho. "There's investigations and procedures they can't get because they're Covid positive.

"Even for rehab - people who are elderly who come in for mobility reasons, physios can't walk them up and down the ward if they're Covid positive.

"So that's another two week delay and those two weeks - for an elderly person who has come in for a specific reason - that's a huge problem.

"Just because we don't have to devote ventilators or high flow oxygen to them doesn't mean it's not an issue for the hospital and for the patient."

Covid infections contracted post-operatively can also slow recovery, placing additional pressure on beds, while a critically injured car crash patient who happens to test positive for Covid can result in a whole area of ICU being sealed off to protect other vulnerable patients.

"That is a huge headache and it will become a bigger headache again," said Lone, adding that the data also misses the "destabilising effect" of Covid on long-term conditions.

"There are a lot of people with underlying health problems where becoming mildly unwell with Covid just tips them over," he said.

"We see that a lot for people with diabetes who don't eat or drink for a few days and end up coming in - they're sick because their diabetes has gone awry, but they wouldn't have been in hospital if they hadn't caught Covid."

HeraldScotland: Booster uptake is high in the older population, but nearly one in 10 adults over 80 is yet to receive a boosterBooster uptake is high in the older population, but nearly one in 10 adults over 80 is yet to receive a booster

For Dr Ho, an immediate priority should be vaccine rollouts in hospital - something never routinely done so far in the pandemic.

She said: "A lot of people on rehab wards are elderly patients in for non-Covid reasons and they may have missed their booster appointment.

"Given that Omicron is so transmissible, and there are more and more patients coming in with Omicron, we need to identify those elderly people in hospital who haven't had a booster but are not currently in because of Covid.

"We really need to target that population."