MORE than six months after the World Health Organisation (WHO) was first notified of a “pneumonia of an unknown cause” affecting patients linked to a seafood market in Wuhan, SARS-CoV-2 continues to puzzle and surprise.

The novel coronavirus which has so far claimed at least 540,000 lives worldwide is now said to be able to linger in the air for hours after infected people have left.

These tiny particles exhaled as infected individuals talk or breathe can float around, especially in enclosed and poorly ventilated environments, according to more than 200 researchers who went public with their concerns this week – accusing the WHO of trying to shut down the evidence.

Previously, transmission of the virus was thought to occur through direct contact – for example being sprayed by larger droplets emitted when an infected person nearby coughs or sneezes, or from touching contaminated surfaces.

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Or even just sitting in a seat previously occupied by someone carrying the virus – whether they were showing symptoms at the time or not. This is why handwashing and cleaning are so important.

The WHO’s technical lead for infection prevention and control, Benedetta Allegranzi, finally conceded yesterday that the organisation cannot rule out evidence emerging of airborne transmission in “crowded, closed, poorly ventilated settings”.

Just days before, she had downplayed the possibility, insisting it was “certainly not supported by solid or even clear evidence”.

The change in tone follows an open letter penned by Lidia Morawska, a expert in aerosol science at Queensland University of Technology, and signed by 239 other scientists.

They wrote that “hand-washing and social distancing are appropriate, but, in our view, insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people”.

Researchers accused the WHO of being “slow and risk-averse in updating its guidance” and allowing “a few conservative voices to shout down dissent” because it could mean more expensive and disruptive control measures.

The issue has attracted more attention from scientists as countries ease their lockdowns by re-opening offices, restaurants, bars and nightclubs.

It has also prompted questions about what level of ventilation is needed in workplaces, schools and nursing homes, and whether ultraviolet light should be used to kill the virus indoors – as sunlight does outdoors.

All this really depends on what proportion of cases are caused by airborne spread, and that is hard to determine.

Some scientists back a “precautionary principle” : until the level of risk is known, follow a path of maximum mitigation such as making face masks mandatory in all indoor settings.

Others are more cautious.

Paul Hunter, a professor of medicine at the University of East Anglia told New Scientist that he was worried it would confuse the public into thinking a face mask was a better protection than washing their hands.

“I think there probably is some airborne transmission,” he added.

“The issue is whether that is important enough to justify additional control measures.”

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All this comes after further doubt was shed on the possibility of herd immunity earlier this week by an updated Lancet study showing that just five per cent of Spain’s population has developed antibodies, despite having one of the world’s highest death tolls.

Given that seroprevalence of at least 60% would be required, it was concluded that “herd immunity through natural infection is not only highly unethical, but also unachievable”.

It is unclear what degree of protection antibodies actually offer anyway, as the study – the largest in Europe to date – also found that over the course of eight weeks, 14% of participants who had initially tested positive for antibodies had no trace of them by the end.

That is not to say they are not immune – they could have cellular immunity instead – but it is another of the virus’ conundrums.

Meanwhile, increasing evidence is also emerging about the long-term consequences of infection with Covid-19.

Studies have found high rates of lung scarring – even those who had mild symptoms – and of blood clots leading to strokes, heart attacks, lung embolisms, and limb amputations. Abnormal blood clotting has even been reported in patients who appear to have recovered.

Neuropsychiatric complications have also emerged, with a study of 125 hospital patients in the UK finding that 39 patients suffered an altered mental state, including encephalitis (an inflammation of the brain which can cause confusion and mobility problems); 10 had a newly diagnosed psychosis; and six had dementia-like symptoms such as short-term memory deterioration. Half of those who experienced altered mental status were younger than 60.

The more we learn, the stranger this coronavirus seems to become.