Until December 2021, the World Health Organization (WHO) was hesitant to officially declare that SARS-CoV-2 is transmitted through the air—a slow-moving response that has drawn both support and critique from scientists and public health experts, Dyani Lewis reports for Nature.

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WHO quietly updates its Covid-19 guidance

On Dec. 23, 2021, WHO quietly updated a page on its website titled 'Coronavirus disease (COVID-19): How is it transmitted?' to state that an individual can be infected "when infectious particles that pass through the air are inhaled at short range", a process also known as "short-range aerosol or short-range airborne transmission."

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According to WHO's website, transmission can occur through "long-range airborne transmission" in indoor settings with poor ventilation or large crowds "because aerosols can remain suspended in the air or travel farther than conversational distance."

Although the statement was "seemingly uncontroversial," Lewis writes, it marked a stark shift for the agency, which had previously tweeted early in the pandemic, "FACT: #COVID19 is NOT airborne." At the time, the agency stated the coronavirus spread primarily through droplets expelled when an individual coughs, sneezes, or speaks—an assumption rooted in decades-old infection-control findings on how respiratory viruses typically pass from person to person.

Notably, WHO did not acknowledge that aerosols could transmit the virus until Oct. 20, 2020. Even then, the agency said this form of transmission was a concern only in certain settings, such as crowded, indoor spaces with poor ventilation. Over the following six months, the agency gradually made updates to its advice to say that aerosols could carry the coronavirus for more than one meter and remain in the air.

Then, WHO issued a statement that placed the virus among a select group of "airborne" infections—a label it reserves for just a few of the world's most virulent pathogens, including measles, chickenpox, and tuberculosis.

Why did it take so long?

Although WHO's latest update mirrors what aerosol and public-health experts encouraged the agency to officially say since the early days of the pandemic, many have criticized the agency for waiting so long to clearly state that SARS-CoV-2 is airborne.

For instance, Nature conducted interviews with dozens of specialists on disease transmission who suggested that WHO's hesitancy to accept and communicate the evidence for airborne coronavirus transmission was rooted in several problematic assumptions about the way respiratory viruses pass from person to person. Others pointed out that the organization prematurely dismissed field epidemiology reports of airborne transmission, while still others critiqued WHO's reliance on a "narrow band of experts, many of whom haven't studied airborne transmission," Lewis writes.

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In addition, many critics have said WHO's reluctance led to similar delays at other national and local health agencies around the world, who were slow to address the airborne threat.

Over the past two years, experts noted that WHO also failed to adequately communicate its changing position. As a result, the agency didn't adequately emphasize the importance of ventilation and indoor masking during the early stages of the pandemic.

According to Lidia Morawska, an aerosol scientist at the Queensland University of Technology in Brisbane, Australia, airborne transmission was "obvious" as early as February 2020.

But not all experts agreed that WHO's slow shift merited critique, Lewis writes. For instance, Dale Fisher, an infectious-diseases physician at the National University Hospital in Singapore and chair of the WHO's Global Outbreak Alert and Response Network steering committee, said he doesn't think that confusion over whether the virus is airborne had significant impact on how the pandemic unfolded. "It's not the cause of the catastrophe we've seen," Fisher said.

And still other researchers defended the agency's response amid the rapidly evolving conditions during the early stages of the pandemic. "I really don't think anybody dropped the ball, including WHO," said Mitchell Schwaber, an infectious-diseases physician at Israel's ministry of health and an external adviser to WHO. "So many assumptions that we had about this virus were proven false. We always ... were learning new things."

Some experts have argued that the agency's decision to classify SARS-CoV-2 as airborne was momentous, Lewis writes. Typically, the agency takes a conservative approach. "What the WHO says is normally based on a consensus of expert advice and opinion," said Christopher Dye, an epidemiologist who served as the scientific adviser to WHO's director-general until 2018.

Although WHO has received harsh criticism for the way it handled the Covid-19 pandemic, some experts have said they were not surprised by the agency's response. WHO "sees its role as certifying the current expert consensus, not (usually) advancing new, tentative knowledge," said Peter Sandman, an independent risk-communications specialist who has worked as a consultant to WHO.

"Individuals and governments and public-health bodies are looking to a WHO [guideline development group (GDG)], not to conjecture. They're looking to a WHO GDG to put out guidance. That everything that we say can be backed by evidence," Schwaber said. (Lewis, Nature, 4/6)