THE good news is that Covid cases in Scotland are now rapidly in decline. The more puzzling aspect is that no one is exactly sure why - or whether it will last.

As daily infection counts rocketed to record levels in August, Nicola Sturgeon warned that she could not rule out re-imposing some restrictions - but held off in the hope that the surge would slow.

It is a gamble that appears to have paid off.

After peaking at an average of more than 6,400 cases per day on September 6, the number of infections being confirmed through testing has plunged by 47 per cent.

It is the first time in the course of the pandemic that a downturn has occurred while there is no physical distancing, no capacity limits on venues, and no businesses - such as gyms or pubs - banned from operating.

Epidemiologists genuinely had no idea what would happen.

Given that the fall has been mirrored be a steady drop in Covid hospital admissions since September 13 and by wastewater surveillance showing a decline in the prevalence of genetic virus material in effluent, the pattern appears to be real as opposed to people simply shunning a test.

READ MORE: Nearly 200 children under 16 in hospital with Covid virus in past four weeks

Professor Sir John Bell, an immunologist and regius professor of medicine at Oxford University, said last week that he believes the UK is now "over the worst" in terms of Covid, while Professor Francois Balloux, a professor of computational biology at University College London, described our current phase as "the beginning of the end"

In England - for reasons that remain a mystery - cases have already plateaued for most of the summer in stark comparison to Scotland's July and August waves, despite both nations having very similar antibody levels and vaccine uptake.

HeraldScotland:

HeraldScotland: In England, cases fell at the beginning of July and have remained fairly flat even after its July 19 Freedom Day and the return of schools in early SeptemberIn England, cases fell at the beginning of July and have remained fairly flat even after its July 19 Freedom Day and the return of schools in early September

Dr Antonia Ho, a consultant and senior lecturer in infectious diseases at Glasgow University, admits to being "surprised" by the sudden decline of Covid cases in Scotland.

She believes that vaccinations may finally have done enough to tip the balance,, however, with the sharpest and earliest fall in case rates evident in the 20 to 24-year-old age group - down 78% since August 28 - followed by those aged 15 to 19.

"I think in that age group it's probably vaccinations," said Dr Ho.

"I wonder, particularly with universities and colleges starting, whether that has driven up vaccination given that in Scotland we are introducing vaccine passports - so if you do want to go out clubbing with your mates you are going to have to get vaccinated.

"Glasgow University has set up vaccination centres within the university and I think that is helping to target that age group."

HeraldScotland: Some universities have established drop-in vaccination centres on campus to boost uptakeSome universities have established drop-in vaccination centres on campus to boost uptake

Since the beginning of September, around 17,000 people aged 18 to 29 have come forward for a first jag and the proportion fully vaccinated - giving maximum protection against Delta - has increased from around 50% to 62%.

Professor Linda Bauld, chair of public health at Edinburgh University, said the impact of universities returning "may be still to come" - but testing and vaccination make campuses and halls of residence safer than they were this time last year, when outbreaks were blamed for kick-starting the autumn wave.

READ MORE: How can Scotland increase vaccine uptake in young people? 

The impact of schools returning and nightclubs reopening was also "settling down" now, she added, following an initial surge coinciding with a sudden increase in people mixing.

HeraldScotland: Graphs by @TravellingTabby Source: Public Health ScotlandGraphs by @TravellingTabby Source: Public Health Scotland

"You had all the nightclubs opening, people coming together who hadn't met, spreading infections and I think now we're seeing that tailing off," she said.

"We've also got a decline in school-age children now in the same way as you see with flus and colds when children go back to primary and nursery school. There was quite a big rise at first.

"The major difference from last year is that what happens when you have so many older adults vaccinated is that what you would normally see - young children, schoolkids, young adults getting infected and then passing it on to people in their 40s and 50s and in turn to older adults - that pattern that we saw in the previous waves, the vaccines are stopping that.

"It's not a complete block, but it's a pretty good block."

Currently, 86% of adults in Scotland are fully vaccinated and 93% have Covid antibodies. By the end of August, roughly 40% of Scots aged 0 to 19 also had antibodies, mostly reflecting prior infection.

HeraldScotland: From Friday, anyone wanting to enter a nightclub will have to have proof they are fully vaccinatedFrom Friday, anyone wanting to enter a nightclub will have to have proof they are fully vaccinated

Although the Delta variant's high transmissibility has pushed herd immunity out of reach, scientists do expect the virus to transition to an endemic state where it continues to circulate at comparatively low levels with occasional "flare ups" in populations where vaccine coverage is lower.

Professor Rowland Kao, chair of veterinary epidemiology and data science at Edinburgh University, thinks we could be approaching this threshold.

"We are in the situation now where the vast majority of the population have either been vaccinated or infected," he said.

"We're reaching the stage where changes in the amount of protective immunity in the population have a substantial effect.

"So we may be at the point where there's enough of that protective immunity around that - under the current the circumstances - it makes the difference between whether we go up or we go down. "

Booster vaccinations and inoculations for 12 to 15-year-olds combined with ongoing measures such as facemasks could help to hold the R number below one, ensuring the epidemic continues to shrink, but Prof Kao warned that this could still be unbalanced by significant changes in behaviour.

HeraldScotland: People continue to report fewer daily contacts compared to August and September 2020 (Scottish Contact Survey, Modelling the Epidemic)People continue to report fewer daily contacts compared to August and September 2020 (Scottish Contact Survey, Modelling the Epidemic)

Even now, despite the lifting of all restrictions, Scots are still interacting with fewer people per day on average than they were this time last year, according to the latest Modelling the Epidemic report.

“Not everyone is going into work every day, so we don’t have a packed office situation. If we head more towards that, which we might, then we could see cases go up again because the contact patterns change."

READ MORE: Why the Delta variant has left herd immunity through vaccination alone impossible

However, the spectre of a significant new variant of the types which derailed Christmas and delayed 2021's much-anticipated return to normality, may also be receding.

Dr Sarah Pitt, a microbiologist and fellow of the Institute of Biomedical Sciences, believes that, in Delta, Covid-19 has discovered its “best self”: an optimum combination of mutations for infectiousness and stability.

She said: "What might be happening is a new variant that wants to come up and be more infectious than Delta might actually be less stable as a virus, therefore it can't outcompete Delta - and that is what we're seeing.

"Every now and then a new virus pops up in Peru or wherever, but then it disappears.

"So Delta seems to have cornered the market.

"Nobody really knows, but we're optimistic because it's been like this for a while."

Other factors could still pile pressure on the NHS.

HeraldScotland: There are concerns that COP26 could drive an upsurge in Covid as well as fluThere are concerns that COP26 could drive an upsurge in Covid as well as flu

COP26 in Glasgow is seen as a potential superspreading event, not only for Covid but for its potential to import influenza.

"It's quite a massive influx of people coming in from all around the world," said Dr Ho.

"If there's an event that was going to seed infections from elsewhere, then that is a very good event to do that.

"I don't see lockdown happening, but there's so many layers before that."

READ MORE: Warning face-to-face GP appointments must be 'rationed for good' to manage demand

Even in a best case scenario, Prof Kao thinks we will have a "slightly bumpy road through winter", particularly if the decline in immunity is sharper than expected.

Current modelling for the UK as a whole has painted a picture Covid infections retreating fairly rapidly during September and October with little Covid activity after November as long as vaccine immunity wanes slowly and boosters are highly effective.

HeraldScotland: Covid hospital admissions have been declining since September 13Covid hospital admissions have been declining since September 13

If the reverse is true, however, the same modelling predicts substantial waves of hospitalisation that would see festive celebrations scuppered again.

"The more we do in terms of physical distancing-type measures, mask wearing, keeping up with hybrid working patterns, the better off we'll be," said Prof Kao.

"It'll help Covid and it'll keep flu down, so we could have an okay winter.

"It's all speculation though because we haven't dealt with Delta in the winter before and we've not dealt with it using these sorts of partial measures.

"It might be that we're still doing the most important things, but we don't know that.

"The chances of it going up are still there, but it could be that it goes into this gentle gradual decline that results in continued circulation but immunity levels high enough so we don't see any surges."