“COVID is airborne”.

This statement is repeated as fact so often in social media hashtags that you might assume it is a matter of scientific consensus, as opposed to one of the most intense debates of the pandemic.

The controversy is not whether airborne transmission ever occurs, but rather how big a problem it is within the overall toll of Covid infections.

In an editorial published in the BMJ on Thursday, Stephanie Dancer - a professor of microbiology at Edinburgh Napier University and an NHS Lanarkshire consultant microbiologist - drew parallels with tuberculosis.

The bacteria which causes TB - Mycobacterium tuberculosis - was famously discovered by the German physician Dr Robert Koch in 1882, but it took a series of animal experiments including guinea pigs exposed to exhaust air from a tuberculosis ward in the 1950s before the scientific community finally accepted airborne transmission as a source of the infection.

READ MORE: After Covid, is it time to make indoor air as clean as tap water?

“It is hoped that SARS-CoV-2 and its proclivity for airborne transmission will be accepted a little quicker than it was for tuberculosis,” adds Prof Dancer.

She is certainly not alone in that wish.

In an open letter to the World Health Organisation in October last year, 239 scientists from 32 countries called on the agency to revise its Covid recommendations in line with evidence they said demonstrated that tiny particles - or “aerosols” - can travel long-distance (two-plus metres) and linger in the air, causing infection when inhaled.

Dr Benedetta Allegranzi, the WHO’s technical lead on infection control, insisted that - while “possible” - such airborne transmission is “certainly not supported by solid or even clear evidence”.

HeraldScotland: Choir practices have been associated with superspreader events apparently involving airborne transmissionChoir practices have been associated with superspreader events apparently involving airborne transmission

The WHO’s position is mirrored in national guidance, including Public Health Scotland updates issued as recently as June 21, which state that the Covid transmission occurs “mainly through close contact with infectious individuals” and/or in “poorly ventilated environments that are not regularly cleaned”.

PHS adds: “Evidence of long-range aerosol transmission is limited.”

This has infuriated those convinced that airborne transmission is actually a significant risk factor.

For them, resistance is part political pragmatism (it’s much easier to ask people to wash their hands and wipe down surfaces than contend with the costlier implications of significant airborne spread, such as mandatory ventilation upgrades), and part scientific Catch-22: are aerosol-related outbreaks rare, or simply very hard to prove?

In real-world settings it is almost impossible to rule out other possible modes of transmission, yet at the same time ethically unviable to design a double-blind control study which would intentionally expose one group of human participants to a known pathogen.

HeraldScotland: The number of Covid positive patients in hospital - a proxy for community prevalence - has doubled from 590 at the end of May to 1,298 by June 26The number of Covid positive patients in hospital - a proxy for community prevalence - has doubled from 590 at the end of May to 1,298 by June 26

With the debate raging on and Covid making yet another comeback, scientists from Bristol University and the UK Health Security Agency sought to unpick the evidence so far.

In their BMJ paper this week - accompanied by Prof Dancer’s editorial - they identified 18 studies worldwide purporting to demonstrate long-distance airborne transmission of Covid.

Only three were rated “high” quality, however.

One involved a New Zealand quarantine hotel in July 2021 where video analysis showed two guests in separate rooms on the same corridor opening their doors simultaneously for a short period of time.

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Both had arrived on separate flights on different days; hotel staff were regularly tested and wore full PPE; and video analysis ruled out close contact between the two guests and “fomite” transmission (where an infection is caused by touching a surface contaminated with Covid droplets).

Yet Guest A - who was asymptomatically infected - appeared to have passed the virus on to Guest B.

Genomic analysis showed their infections matched (pointing to Guest A as the source) and, other than the brief door opening, the only other possible routes of transmission were via the hotel’s inter-room ventilation systems.

HeraldScotland: Restaurant settings have also be linked with airborne Covid spread between customers at separate tablesRestaurant settings have also be linked with airborne Covid spread between customers at separate tables

A second outbreak, which occurred in a South Korean restaurant in June 2020, involved three diners at three separate tables: Diner A was pre-symptomatic for Covid at the time but spent five minutes sitting 6.5 metres from Diner B, and 4.8 metres from Diner C for 21 minutes.

All three subsequently tested positive for Covid infections which matched the same genomic cluster.

Again, video surveillance ruled out close contact and fomite transmission, but noted that the restaurant’s air circulation units may have contributed to virus spread via “directional air flow from the primary to secondary cases combined with lack of air replacement”.

READ MORE: The inside story of Scotland's child hepatitis outbreak

The final high-quality study, from Sydney, Australia, suggested that an infected choir member who sang at four church services appeared to have passed the virus on to as many as 12 others after sitting within one to 15 metres of them.

At a time when some scientists and campaigners are calling for venues such as shops and restaurants to display air quality ratings in a similar way to food hygiene certificates, it is notable that even some healthcare settings fall short.

For example, national guidance on ventilation recommends six air changes per hour in hospital areas used for suspected Covid patients, yet according to a recent inspection the Queen Elizabeth University Hospital in Glasgow - where windows cannot open - the mechanical ventilation system provides just three air changes per hour.

As such, they are simply required to keep patients two metres apart, and provided with facemasks and handwashing facilities.

HeraldScotland: Positive Covid among hospital inpatients, March 1 2020 to June 5 2022 (dark blue = definite hospital onset, where positive specimen taken 15 days post-admission)Positive Covid among hospital inpatients, March 1 2020 to June 5 2022 (dark blue = definite hospital onset, where positive specimen taken 15 days post-admission)

Could issues like this - not unique to the QEUH - partly explain why, despite all the cleaning, distancing and PPE measures in place, almost as many patients have caught Covid in hospital this year alone (3,955 definite hospital-onset cases by June 5) compared to the whole of the previous two years of the pandemic (5,012 such cases)?

Maybe. But don't expect an outbreak of scientific consensus anytime soon.