COVID-19 airborne transmission: What we know so far

It's official: COVID-19 spreads through the air

There’s a new understanding of SARS-CoV-2: the virus spreads primarily through the air. "Primarily" is the operational word. This recognition has been dubbed as a “quiet revolution”, with far-reaching implications.

Previous studies have demonstrated that buildings had been associated with spread of infectious diseases, such as outbreaks of measles, influenza, and Legionella.

Given the airborne nature of SARS-CoV-2, global health officials now recognise what scientists have been saying for more than a year — that COVID-19 is indeed airborne. Here’s what we know, so far:

What’s new?

Transmission of SARS-CoV-2 by air is a body of scientific knowledge now adopted by both the World Health Organisation (WHO) and the US Centres for Disease Control and Prevention (CDC). In so doing, they have effectively dropped the “droplet dogma”, i.e. the insistence, that COVID-19 infections are primarily triggered by droplets, instead of airborne.

A comprehensive understanding about airborne COVID-19 infections, however, goes back at least a year. For example, the rampant contagion via in-door settings in hotels, call centres, restaurants, meat-processing plants or quarantined cruise ships (even if the guests didn’t come out of their cabins) had been cited.

“Super spreaders” events traced to a church choir in Australia, elevator use in China, and a conference in the US are well documented. Since then, evidence had been piling up, making them increasingly tough to ignore.

How long does the COVID-19 virus survive on surfaces?

The timeline is not clear, based on studies, according to WHO. However, most likely it behaves like other coronaviruses. Research shows coronaviruses can survive on surfaces for a few hours up to several days, depending on conditions, like the type of surface, temperature or humidity.

Droplets vs aerosols: What’s the difference?

In reality a person who coughs, sneezes or talks can expel both droplets (>5 microns) and aerosols (< 5 microns). So this distinction is artificial, and only exists for discussion purposes. However, the “primacy” of airborne transmission — the key word is primacy — carries with it huge implications for infection control protocols.

The 5-micron threshhold for a particle's ability to stay aloft is an archaic trap. It's a product of a decades-old dogma in the infectious disease research fraternity which has now been discredited, thanks to modern-day understanding of atmospheric physics.

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While large droplets settle to the ground almost immediately after they are emitted within a few feet of an infected person, fine virus-laced respiratory particles can become airborne for hours on end and spread farther away. The 5-micron threshhold for a particle's ability to stay aloft is an archaic trap.

It's a product of a decades-old dogma in the infectious disease research fraternity which has now been discredited, thanks to modern-day understanding of atmospheric physics. In fact, latest air particulate studies show that particles of many sizes can stay in the air, travel far and be inhaled.

The size of the largest coronavirus “virion”, an infinitesimal 0.14 micron (or about 140 nanometers) is about 1/500th of a human hair. In general, the size threshold that dictates whether droplets fall or float is actually 100 microns, not 5 microns, according to aerosol scientists studying infectious diseases. In short, larger droplets travel farther than what we’ve been told. The battle has been fought during virtual meetings as well as on social media. On March 29, 2020 the agency's Twitter feed declared: "Fact: #COVID is NOT airborne."

Haven’t scientists been saying that COVID is transmitted via aerosol all along?

Yes. In general there were two opposing groups of scientists — one pushed the aerosol-as-primary-mode-of-COVID-transmission hypothesis, and another group who said it’s primarily through droplets. As COVID-19 made its deadly run across the world, it’s become untenable to hold on the droplets hypothesis.

By now, it’s become widely understood that SARS-CoV-2 is mainly transmitted from the exhaled respiratory aerosols of infected individuals. Larger droplets (>100 μm) can settle on surfaces quickly due to gravitational forces within 6 feet. In October 2020, scientists showed evidence that that people emit “100 times more smaller aerosols” (<5 μm) — when talking, breathing, and coughing.

This study, published by the National Academies of Sciences, Engineering, and Medicine, also demonstrated that smaller aerosols can stay aloft for anywhere from 30 minutes to hours and travel well beyond 6 feet.

What’s the evidence of airborne COVID-19 transmission?

In general, there are two sets. The first is the background of SARS-CoV-2 itself: other coronaviruses — the MERS (Middle Eastern respiratory sydrome) virus and SARS (severe acute respiratory syndrome, now known as SARS-CoV-1) — are primarily spread via aerosols. Both are accepted as being airborne.

And earlier this 2021, The Lancet COVID-19 Commission described high-profile SARS-CoV-2 outbreaks (also known as “super-spreader events) across multiple space types, such as restaurants, gyms, choir practice, schools, buses, cruise ships.

The common denominator: time indoors and low levels of ventilation — even when people remained physically distanced. Moreover, the journal Lancet has cited 10 scientific reasons that bolsters airborne transmission of SARS-CoV-2.

Kimberly Prather, an expert in airborne viruses and professor at University of California in San Diego, stated in a tweet: “Sadly, if anyone is arguing against the fact this virus is airborne or adding arguments that include the word ‘nuance’, you should seriously question their motivation. This virus travels in the air. Not rocket science. Clean the air. Filtration/ventilation/masks/distance.”

What’s changed?

Now, health authorities taking cues from WHO and CDC could also change their messaging. When the pandemic started, the WHO said the virus was spread by “large droplets” — thus handwashing was promoted and mask-wearing was even discouraged initially.

The result: A false narrative emerged, washing of hands and disinfecting surfaces (even groceries or bottles) were extolled. Some scientists, among them occupying key positions in the WHO, formed part of the so-called “airborne deniers club”, who point to a "lack of evidence for the airborne thesis.

What is the significance of this shift from droplets to aerosols?

The implications are huge. Citing the evidence for airborne transmission, researchers published on April 21, 2021 in JAMA outlining proposed changes in indoor air control protocols.

“With SARS-CoV-2, the majority of outbreaks involving 3 or more people have been linked with time spent indoors, and evidence confirms that far-field airborne transmission (defined as within-room but beyond 6 feet) of SARS-CoV-2 is occurring,” stated Harvard researcher Joseph G. Allen and his colleague Andrew M. Ibrahim, who is with an indoor air expert at Taubman College of Architecture & Urban Planning, University of Michigan in Ann Arbor.

“With respect to engineering controls, an important flaw exists in how most buildings operate in that the current standards for ventilation and filtration for indoor spaces, except for hospitals, are set for bare minimums and not designed for infection control.”

Today, there are increasingly louder calls for improving air ventilation rates, though there has been limited guidance on specific ventilation and filtration levels. To limit what they researchers call “far-field airborne transmission of SARS-CoV-2”, they suggested increasing outdoor air ventilation and enhancing filtration, with set targets.

This acceptance by official health bodies carries a number of must-dos, according to researchers, who propose a redesign, or overhaul of ventilation systems. There are two-pronged infection control strategies advanced:

What’s with the “droplet dogma"?

In 1854, John Snow, a doctor, wrote about his work showing the link between faeces-laden drinking water and cholera in the UK. Cholera was one of the world’s deadliest diseases, blamed for causing serial epidemics.

For most of the 19th century, it was believed, without evidence, that cholera was caused by “bad air” or “bad smells” from rotting organic matter. Snow’s study proved that to be a mistaken — and deadly — understanding, which eventually led to wide-ranging changes in sewage treatment and sanitation engineering.

The so-called “droplet dogma” first emerged 90 years ago, first articulated by William Wells in 1930 for tuberculosis. It holds that contagion is largely limited to the distance covered by droplets that are larger than five to 10 microns in size. But subsequent studies challenged that view, and proved it to be wrong.

TIMELINE: Droplets vs aerosols

March 29, 2020:

@WHO tweets: "Fact: #COVID is NOT airborne."

July 4, 2020:

In an open letter, 239 scientists from 32 countries called on the WHO agency to acknowledge the high risk airborne COVID-19 transmission as a potential driver of the pandemic. The WHO stopped short of revising its messaging, citing a lack of "definitive" evidence. WHO officials insisted droplets are the key route of SARS-COV-2 transmission.

July 8, 2020:

The WHO's technical lead on the pandemic, Maria Van Kerkhove, said the agency's guidelines on airborne transmission are primarily focused on hospitals. Some experts say that statement didn’t go far enough to inform the public about the potential threat of the virus through the air. WHO stated then that SARS-CoV-2 transmission is “mainly between people who are in close contact with each other, typically within 1 meter,” or about 3 feet.

April 9, 2021:

Dr. John Conly, top adviser to WHO and chair of the WHO’s Infection Prevention and Control Research and Development Expert Group for COVID-19, said that N95 masks can cause “harms” — including acne — and pointed to a lack of “scientific evidence” for airborne transmission as the predominant means of COVID-19 infection.

April 25, 2021:

The US Centers for Disease Control and Prevention (CDC) changes its messaging, stating that the risk of surface transmission of COVID-19 is “lower” compared to airborne transmission — and people who obsessively disinfect surfaces may be doing more harm than good.

April 28, 2021:

#COVIDisAirborne starts trending on Twitter. It's symptomatic of the debate raging between scientists, notably within the WHO, and those outside the organisation who believe the droplet view is outdated.

April 30, 2021:

WHO amends COVID-19 advisory: "The virus can spread from an infected person’s mouth or nose in small liquid particles when they cough, sneeze, speak, sing or breathe. These particles range from larger respiratory droplets to smaller aerosols…

“Current evidence suggests that the virus spreads mainly between people who are in close contact with each other, typically within 1 metre (short-range). A person can be infected when aerosols or droplets containing the virus are inhaled or come directly into contact with the eyes, nose, or mouth.”

“The virus can also spread in poorly ventilated and/or crowded indoor settings, where people tend to spend longer periods of time. This is because aerosols remain suspended in the air or travel farther than 1 metre (long-range).”

May 17, 2021:

A World Health Organisation has reportedly dropped the “droplets dogma”, embracing what experts have argued for more than a year, i,e. that the coronavirus primarily spreads through the air.