The world may have moved on, but Covid is continuing to circulate and evolve.

The SARS-CoV2 virus has been back in the news in recent weeks amid evidence that yet another mutated strain of Omicron - this time known as XEC - is on track to become the next dominant variant.

Like its predecessors, it appears to have a "slight transmission advantage" enabling it to spread more easily and dodge protection from antibodies - the body's first line of defence against infection.

READ MORE: 

There are 82 confirmed cases in the UK, although the true figure will be much higher as surveillance of the virus has long since been cut back to a few sentinel GP surgeries and hospital testing of symptomatic patients.

Preliminary analysis suggests that XEC will become the dominant variant in Europe and the UK some time in October.

This year's winter rollout will use vaccines last updated to combat the BA.4 and BA.5 offshoots of Omicron which became dominant in summer 2022, but there is nothing to suggest that the vaccines will be less effective when it comes to protecting against serious disease from infections caused by XEC.

At this point, some scientists are referring to the current strains in circulation as the "grandchildren or great-grandchildren" of Omicron: still strongly related, but drifting further away genomically.

When Omicron first exploded at the end of 2021, it changed the game.

It had more than 50 mutations distinguishing it from the original "wild type" which emerged in China late 2020, mostly concentrated on the spike protein.

This made it hugely more contagious.

Whereas the original form of the virus was estimated to have a reproductive number of three - meaning that each infected person would pass it on to three others in a population which had zero prior exposure and where there were no mitigations in place, such as social distancing - Omicron is estimated to have an R number of eight.

Scotland's chief medical officer, Professor Sir Gregor Smith, arrives to give evidence at the UK Covid inquiry on Wednesday September 25 2024Scotland's chief medical officer, Professor Sir Gregor Smith, arrives to give evidence at the UK Covid inquiry on Wednesday September 25 2024 (Image: PA) Each of its descendants has nudged that number up slightly.

The spread of the virus is curtailed now only by much higher levels of population immunity since most people have been vaccinated and infected multiple times already.

The question of transmissibility - in particular, how Covid spreads and what we should do to stop it - has also been back on the agenda over the past few weeks after the UK Covid Inquiry resumed for hearings into Module Three: the impact of Covid-19 on healthcare systems across the four nations.

Prof Susan Hopkins, the chief medical advisor for the UK Health Security Agency, caused some controversy when she told the inquiry on September 18 that the evidence for using FFP3 respirator masks instead of surgical masks to reduce the spread of the virus in healthcare settings was "weak".

This contradicted studies such as research by Cambridge University Hospitals NHS Foundation Trust which observed that when healthcare staff working on its Covid wards switched, in December 2020, from wearing surgical masks to FFP3 respirators the number of Covid infections among this group rapidly fell to the same levels experienced by staff on non-Covid wards.

This prompted claims that they were "most likely" providing 100% protection and has continued to inform calls by some campaigners to make high-grade masks a requirement in settings such as hospitals to protect people with health conditions that make them more vulnerable to Covid, and less responsive to vaccines.

Against that backdrop, Prof Hopkins' evidence caused a stir.

Even Baroness Heather Hallett, the inquiry's chair, seemed sceptical.

Holding up the two different types of masks, she noted that the surgical one was "flimsy" with "lots of gaps".

The FFP3 must offer "so much more protection", added Baroness Hallett.

For Prof Hopkins, however, the question was which worked better "in real life".

The FFP3 masks are fit tested in a way that means they are effectively sealed against the wearer's face.

Prolonged use resulted in some people developing ulcers, struggling to breathe, or becoming dehydrated, said Prof Hopkins.

"We have to talk about how it's worn for a 12-hour shift, how it's worn repeatedly day after day," she told the inquiry, acknowledging that users were more likely to remove FFP3s than surgical masks due to discomfort.

She added: "They work really well in the lab, the laboratory [evidence] is really clear, but when you look at them in practice...does what happens in the laboratory actually work in practice, in real life?

"That's the challenge that we have."

Evidence to the UK Covid inquiry reignited a debate about FFP3 respirator masks in healthcareEvidence to the UK Covid inquiry reignited a debate about FFP3 respirator masks in healthcare (Image: Getty) Not everyone agrees with Prof Hopkins' assessment, but the evidence was important in the context of another hot button topic of the pandemic: how much should we worry about airborne transmission?

If droplets - from coughs, sneezes, and close contact with an infected person - were the main mode of transmission, surgical masks should be effective.

But if long-range transmission via aerosols which can remain suspended in the air for hours after an infected person breathes out is a major source of spread, then respirator masks become much more important.

On March 28 2020, the World Health Organisation put out what is now - in retrospect - a toe-curlingly inaccurate social media post, stating: "Fact: Covid 19 is not airborne".

It was a mantra that shaped public health responses to Covid during the first wave, including in Scotland, where handwashing was emphasised but the significance of social distancing, facemasks, and ventilation was neglected until the WHO backtracked in July 2020 conceding that aerosol transmission "could not be ruled out".

READ MORE: 

Giving evidence to the inquiry on Wednesday, Scotland's chief medical officer Professor Sir Gregor Smith said the WHO's equivocations had been "unhelpful" because it created a level of uncertainty that "made it more difficult to get the necessary levels of investment in place and to convince everybody that investing in ventilation was something that was important".

By December 7 2021, when the Scottish Government's Cabinet met to discuss the escalating Omicron emergency, Prof Smith said the evidence for aerosol transmission "was very real", but the extent to which it contributes to total infections "was less clear".

Today, some scientists believe airborne spread is behind the majority of infections.

But - like Covid itself - the arguments rage on.