COVID-19 transmission more indoors than outside: Lancet study

A new assessment in the medical journal Lancet has found "consistent, strong evidence" that the SARS-CoV-2 virus is predominantly transmitted through the air.

Therefore, public health measures that fail to treat the virus as predominantly airborne leave people unprotected and allow the virus to spread, according to six experts from the UK, USA and Canada, including Jose-Luis Jimenez, a chemist at the Cooperative Institute for Research in Environmental Sciences (CIRES) and the University of Colorado Boulder.

"The evidence supporting airborne transmission is overwhelming, and evidence supporting large droplet transmission is almost non-existent," Jimenez said. "It is urgent that the World Health Organization and other public health agencies adapt their description of transmission to the scientific evidence so that the focus of mitigation is put on reducing airborne transmission."

The team of experts, led by the University of Oxford's Trish Greenhalgh, reviewed published research and identified 10 lines of evidence to support the predominance of the airborne route.

At the top of their list: Super-spreader events such as the Skagit Choir outbreak, in which 53 people became infected from a single infected case. Studies have confirmed these events cannot be adequately explained by close contact or touching shared surfaces or objects.

Moreover, transmission rates of SARS-CoV-2 are much higher indoors than outdoors, and transmission is greatly reduced by indoor ventilation.

The team highlighted that silent (asymptomatic or pre-symptomatic) transmission of SARS-CoV-2 from people who are not coughing or sneezing accounts for at least 40 per cent of all transmission. This silent transmission is a key way COVID-19 has spread around the world, "supporting a predominantly airborne mode of transmission," according to the assessment. The researchers also cited work demonstrating long-range transmission of the virus between people in adjacent rooms in hotels; people who were never in each other's presence.

By contrast, the team found little to no evidence that the virus spreads easily via large droplets, which fall quickly through the air and contaminate surfaces.

The new work has serious implications for public health measures designed to mitigate the pandemic. First, "droplet measures" such as handwashing and surface cleaning, while not unimportant, should be given less emphasis than airborne measures, which deal with inhalation of infectious particles suspended in the air.

If an infectious virus is primarily airborne, someone can potentially be infected when they inhale aerosols produced when an infected person exhales, speaks, shouts, sings, or sneezes. So airborne control measures include ventilation, air filtration, reducing crowding and the amount of time people spend indoors, wearing masks whenever indoors and attention to mask quality and fit, when working in contact with potentially infectious people.

‘’It is quite surprising that anyone is still questioning whether the airborne transmission is the predominant transmission pathway for this virus or not," said co-author Professor Kimberly Prather, an aerosol scientist from the University of California San Diego. "Only by including inhalation of aerosols at both close and long range can we explain the many indoor outbreaks that have occurred around the globe. Once we acknowledge this virus is airborne, we know how to fix it. There are many examples of places that have fared much better by acknowledging this virus is airborne from the start. The world needs to follow their lead as soon as possible."

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