Why WHO changed the definition of "airborne transmission" in light of the pandemic

After the confusion of 2020, the WHO has finally changed its definition of how diseases can spread through the air. But what is the new definition—and what should happen next? 

What happened?

In the spring of 2020, as COVID-19 began to spread, there was confusion among scientists, doctors, public health experts and others. Many have argued that the new virus is airborne, but the World Health Organization (WHO) has refused to use the terms "airborne" or "aerosol" in the context of COVID-19 until 2021. This had implications as the world debated wearing masks (and what types of masks were appropriate) and whether closed spaces affected infection.

Now, four years later and after two years of discussion among experts, the WHO has changed its definition of "airborne" spread of infectious pathogens in hopes of avoiding the confusion and miscommunication that characterized the first year of the pandemic and hampered attempts to control the spread of the virus. p>

What was the previous definition?

Until now, WHO called a pathogen airborne only if it was capable of moving through the air in particles smaller than 5 microns and infecting people at a distance of more than 1 meter. Only a small number of pathogens, notably measles and tuberculosis, met this definition. Most respiratory viruses, according to WHO, were transmitted by “droplet transmission,” when droplets are released by a sick person when they cough or sneeze and land in the eyes, mouth or nose of another person.

These definitions have implications for disease control. Stopping the spread of smaller particles requires containment rooms, N95 respirators and other protective measures that are more difficult and expensive than hand washing and social distancing. The question also arises which of these measures are only needed in hospitals and which are also required in other places.

During the pandemic, experts argued that scientists and engineers had already demonstrated that the WHO's categorization by distance and particle size was flawed and that infectivity could be due to a variety of factors, such as clouds of particles that could infect people over short distances or more large particles that can remain in the air for a long time. Others argued that a short distance is not evidence that the disease is spread only by coughing and sneezing, since droplets can also be spread by breathing or talking.

What is the new WHO definition?

The new report clearly divides pathogen transmission into those that occur through contact (with people or surfaces) and “airborne transmission.”

Airborne transmission is divided into two subcategories: "direct deposition", where droplets reach the mucous membranes of another person's mouth, eyes or nose (essentially the previous WHO definition of airborne transmission), and "airborne transmission or inhalation", where the droplets are inhaled. It is this second subcategory that represents a significant change. The key point is that the entire definition is independent of the size or distance of the droplets.

What does this mean for future epidemics and pandemics?

Agreement on the new definitions has already been reached with the Africa Centers for Disease Control and Prevention, the Chinese Center for Disease Control and Prevention, the European Center for Disease Prevention and Control and the US Centers for Disease Control and Prevention. However, what this means for a future epidemic or pandemic remains to be seen.

The WHO report makes no recommendations on how these new definitions should or could impact prevention or control policies, noting only that reducing the spread of small infectious respiratory particles will require "airborne precautions" such as masks and isolation rooms. The report said there was no consensus on prevention and control.

Who was on the advisory group?

The advisory group included about 50 scientists, doctors and engineers. They sought to emphasize the importance of involving engineers and environmentalists, not just public health and medical experts.

What did they say about the new definition?

"We can now use the word 'airborne' to communicate with the public," said Lindsay Marr, an environmental engineer at Virginia Tech in the US and a member of the advisory group. “In the past, public health officials avoided the word, and people didn’t understand why.” She noted that some of the language is still “awkward,” but the important thing is that the science is correct.

Farrar said the key is to build a new definition based on clear experimental data. He noted that there is still debate about whether influenza, despite more than 100 years of study, is transmitted through the air or not. "We know a certain amount, but we're not absolutely sure," Farrar said. “This is the kind of work we desperately need for flu.”