PEOPLE should be asked to wear face coverings when attending large outdoor gatherings such as sporting events to prevent Covid super-spreading, researchers have argued.

Writing as part of a debate in the BMJ today, clinicians from California and Switzerland say the policy makes sense in settings where it is "difficult to maintain physical distance for prolonged periods".

Although they acknowledge that the estimated risk of transmission is "four to 20 times more likely" indoors than outdoors, they conclude that mandating facemasks outdoors when people are mixing with several other household groups helps to make it a normalised behaviour.

HeraldScotland: Black Lives Matter rallies in the US in 2020 were not linked to a spike in cases despite predictionsBlack Lives Matter rallies in the US in 2020 were not linked to a spike in cases despite predictions

The authors - Babak Javid of the Division of Experimental Medicine at the University of California (UCal) and Dirk Bassler and Manuel B Bryant of University Hospital Zurich - note that the outcomes of mass outdoor events have varied.

They write: "Summer 2020 witnessed worldwide mass protests in support of the Black Lives Matter movement.

"At the time, many experts and pundits warned that such protests may fuel large transmission clusters for Covid-19, but these fears were not realised.

"By contrast, the mass outdoor Sturgis Motorcycle Rally in South Dakota, USA, is considered to have been the trigger for a huge superspreading-type event that resulted in a devastating chain of Covid-19 transmission and disease.

"One proposed reason for the observed differences is that the Sturgis Rally was associated with lower compliance with non-pharmaceutical interventions (NPIs), such as mask wearing and physical distancing, that are associated with decreased transmission risk."

HeraldScotland: A woman wearing a Trump flag dances while watching a band perform during the motorcycle rally in August 2020. The event was subsequently linked to thousands of infectionsA woman wearing a Trump flag dances while watching a band perform during the motorcycle rally in August 2020. The event was subsequently linked to thousands of infections

The motorcycle rally, which brought together 462,000 biker enthusiasts in August, was subsequently linked to 260,000 coronavirus cases between August 2 and September 2 by researchers at San Diego State University.

The authors also note that in the San Francisco Bay Area of California, which implemented one of the earliest and strictest public mask mandates, has recorded the lowest mortality risk from Covid-19 of any large city in the US.

READ MORE: Medics call for facemasks to be worn outdoors amid variant fears 

They add: "In this region, wearing masks outdoors is mandated if you encounter people from outside of your household.

"Importantly, such a mandate is associated with extremely high compliance with mask wearing not only outdoors but also indoors, where most transmission risk exists."

However, Dr Muge Cevik, an expert in infectious diseases and virology at St Andrews University, argued that the Sturgis Rally had "sustained and multi-day indoor components", with epidemiological investigations finding links to restaurants and workplaces.

READ MORE: Why an apparent rise in Covid cases isn't always what it seems when virus rates are low

Dr Cevik, who is a member of the UK Department of Health's NERVTAG (New and Emerging Respiratory Virus Threats Advisory Group) and advises Scotland's Chief Medical Officer on Covid-19, said that fewer than 10 per cent of known Covid cases involved outdoor transmission.

She added: "Given the low risk of transmission outdoors,recommendations or mandates for outdoor masking may seem arbitrary, affecting people’s trust and sustained energy to engage in higher yield interventions, such as indoor mask use or staying home if sick.

"Blanket outdoor mask recommendations may also confuse the public about the true nature of the relative risk and could distract from indoor settings with a much greater transmission risk."

HeraldScotland: Mandating masks in outdoor settings would 'confuse' public about risk, said Dr Muge CevikMandating masks in outdoor settings would 'confuse' public about risk, said Dr Muge Cevik

There is no requirement to wear masks outdoors in Scotland, and there is also growing evidence that vaccinations are curbing transmission.

A study on Wednesday from Public Health England found that people given single dose of either the Pfizer or AstraZeneca vaccines - but who nonetheless became infected three weeks later - were 38% and 49% less likely, respectively, to pass the virus on than unvaccinated people.

READ MORE: Mass home testing for Covid 'misguided' and could lead to increased transmission

It comes after 22 scientists, including the director of Oxford University's Centre for Evidence Based Medicine Professor Carl Heneghan and geneticist and health data scientist Professor Anthony Brookes, wrote an open letter to the Daily Telegraph at the weekend calling for face coverings and social distancing to be abandoned in the UK "no later than June 21" - indoors and outdoors.

They said it would no longer be necessary as vaccines "will reduce Covid deaths by around 98 per cent and serious illness by 80-85%" once vulnerable groups have had both doses.

Their letter was widely criticised as irresponsible by other scientists, who insisted that mask wearing indoors was a simple and necessary precaution which should continue - possibly until 2022.