Emerging evidence of airborne spread of Covid-19

There is "evidence emerging" of the airborne spread of the novel coronavirus, which has infected more than 11.8 million people and killed some 543,000 across the world.

The World Health Organization on Tuesday acknowledged the risk after a group of scientists urged the global body to update its guidance on how the respiratory disease passes between people.

"We have been talking about the possibility of airborne transmission and aerosol transmission as one of the modes of transmission of Covid-19," Maria Van Kerkhove, technical lead on the pandemic at the WHO, told a news briefing.

The WHO has previously said the SARS-CoV-2 virus that causes the Covid-19 respiratory disease spreads primarily through small droplets expelled from the nose and mouth of an infected person that quickly sink to the ground.

But in an open letter to the Geneva-based agency, published on Monday in the Clinical Infectious Diseases journal, 239 scientists in 32 countries outlined evidence that they say shows floating virus particles can infect people who breathe them in.

Because those smaller exhaled particles can linger in the air, the scientists are urging WHO to update its guidance.

Risk of transmission

Speaking at Tuesday's briefing in Geneva, Benedetta Allegranzi, the WHO's technical lead for infection prevention and control, said there was evidence emerging of airborne transmission of the coronavirus, but that it was not definitive.

" ... the possibility of airborne transmission in public settings, especially in very specific conditions, crowded, closed, poorly ventilated settings that have been described, cannot be ruled out," she said.

"However, the evidence needs to be gathered and interpreted, and we continue to support this."

Any change in the WHO's assessment of risk of transmission could affect its current advice on keeping 3.3 feet of physical distancing. Governments, which rely on the agency for guidance policy, may also have to adjust public health measures aimed at curbing the spread of the virus. Others have doubled the number to recommend nearly 2 metres distance.

Van Kerkhove said the WHO would publish a scientific brief summarising the state of knowledge on modes of transmission of the virus in the coming days.

"A comprehensive package of interventions is required to be able to stop transmission," she said.

"This includes not only physical distancing, it includes the use of masks where appropriate in certain settings, specifically where you can't do physical distancing and especially for healthcare workers."

Small but deadly virus particles

In the letter published this week in the journal Clinical Infectious Diseases, two scientists from Australia and the US wrote that studies have shown “beyond any reasonable doubt that viruses are released during exhalation, talking and coughing in microdroplets small enough to remain aloft in the air”.

That means people in certain indoor conditions could be at greater risk of being infected than was previously thought.

WHO has been criticized in recent weeks and months for its seeming divergence from the scientific community.

The letter stated that the issue of whether or not Covid-19 was airborne was of “heightened significance” as many countries stop restrictive lockdown measures.

The authors cited previous studies suggesting that germs closely related to the new virus were spread via airborne transmission.

They said “there is every reason to expect” that the coronavirus behaves similarly. 

They also cited a Washington state choir practice and research about a poorly ventilated restaurant in Guangzhou, China, each of which raised the possibility of infections from airborne droplets.

“We are concerned that the lack of recognition of the risk of airborne transmission of Covid-19 and the lack of clear recommendations on the control measures against the airborne virus will have significant consequences,” the scientists wrote. “People may think they are fully protected by adhering to the current recommendations but in fact, additional airborne interventions are needed.”

READ MORE: Mutated and possibly airborne coronavirus

Understanding the new virus

Scientists around the world have been working furiously to understand the new virus. The US Centers for Disease Control and Prevention says it is thought to mainly jump from person to person through close contact, but adds: “We are still learning about how the virus spreads.”

Martin McKee, a professor of European Public Health at the London School of Hygiene and Tropical Medicine who was not linked to the letter, said the scientists' arguments sounded “entirely reasonable”.

“Part of the problem is that everybody at WHO was moving with the paradigm of influenza, even though we know there are lots of differences between influenza and coronaviruses,” he said.

McKee noted that with Britain's recent reopening of its pubs, restaurants and salons, the possibility of airborne coronavirus transmission might mean stricter interventions are needed indoors, including more mask-wearing and continued physical distancing.

“We're getting accumulating evidence about super-spreading events happening in indoor spaces where there are large numbers of people in confined spaces,” he said.

“Many of these are in exactly the circumstances that governments now want to open up.”