- Coronavirus airborne spread was a hot topic this week, as hundreds of scientists urged the World Health Organization to acknowledge the importance of aerosols in COVID-19 transmission.
- The WHO said earlier this week that the emerging evidence does support airborne transmission and announced that it will release updated guidelines.
- The new document does elaborate on the risk of airborne spread, but says it can happen only under certain conditions and that droplet spread and human contact are the primary methods of infection.
There was a lot of talk about the way the novel coronavirus spreads earlier this week. A group of 239 scientists urged the World Health Organization (WHO) to acknowledge the coronavirus is airborne. In other words, the virus can linger in the air after infected individuals expel tiny particles that turn into aerosols.
Reports at the time indicated that the WHO would be resistant to change, and will continue to promote droplets as the main avenue of contracting the disease. Droplets are bigger particles of saliva than the ones that become aerosols, and they tend to land on surfaces several feet away from the person who coughs, sneezes, or talks. WHO did acknowledge the airborne spread risk for COVID-19, but referred to the available data as “emerging evidence,” and said the matter would require more study.
The organization finally did issue new guidelines for COVID-19 transmission, which continue to stress the importance of droplet transmission, with handwashing and hygiene being the main ways to reduce risk. Airborne transmission is mentioned, with the WHO saying it can happen in certain conditions.
The updated scientific brief has been published on the WHO website, and contains ten pages that detail the transmission of the SARS-CoV-2 virus, and the “implications for infection prevention precautions.”
“Airborne transmission is defined as the spread of an infectious agent caused by the dissemination of droplet nuclei (aerosols) that remain infectious when suspended in air over long distances and time,” says the paper in the section dedicated to airborne transmission. But the WHO makes it clear that aerosol transmission can happen only in certain situations. “Airborne transmission of SARS-CoV-2 can occur during medical procedures that generate aerosols (“aerosol generating procedures”). WHO, together with the scientific community, has been actively discussing and evaluating whether SARS-CoV-2 may also spread through aerosols in the absence of aerosol generating procedures, particularly in indoor settings with poor ventilation.”
The WHO links to experiments that prove aerosol transmission is possible, but notes that “much more research is needed given the possible implications of such route of transmission.” What that means is that the WHO will not issue face mask mandates or require ventilation policies for indoor settings to reduce the risk of transmission.
That said, airborne transmission gets its own bullet point in the key points section of the brief, an indication that the WHO is taking the matter seriously:
Airborne transmission of the virus can occur in health care settings where specific medical procedures, called aerosol generating procedures, generate very small droplets called aerosols. Some outbreak reports related to indoor crowded spaces have suggested the possibility of aerosol transmission, combined with droplet transmission, for example, during choir practice, in restaurants or in fitness classes.
The WHO also says that face masks should be advised in certain conditions, stopping short of saying that face coverings should be employed at all times (emphasis ours):
Given that infected people without symptoms can transmit the virus, it is also prudent to encourage the use of fabric face masks in public places where there is community transmission and where other prevention measures, such as physical distancing, are not possible. Fabric masks, if made and worn properly, can serve as a barrier to droplets expelled from the wearer into the air and environment. However, masks must be used as part of a comprehensive package of preventive measures, which includes frequent hand hygiene, physical distancing when possible, respiratory etiquette, environmental cleaning and disinfection. Recommended precautions also include avoiding indoor crowded gatherings as much as possible, in particular when physical distancing is not feasible, and ensuring good environmental ventilation in any closed setting.
The WHO explains that infected people with symptoms can infect others primarily through droplets and close contact. The WHO says that asymptomatic carriers and presymptomatic patients can also spread the virus.
The paper also acknowledges that questions remain about transmission that need to be addressed:
Many unanswered questions about transmission of SARS-CoV-2 remain, and research seeking to answer those questions is ongoing and is encouraged. Current evidence suggests that SARS-CoV-2 is primarily transmitted between people via respiratory droplets and contact routes – although aerosolization in medical settings where aerosol generating procedures are used is also another possible mode of transmission – and that transmission of COVID-19 is occurring from people who are pre symptomatic or symptomatic to others in close contact (direct physical or face-to-face contact with a probable or confirmed case within one meter and for prolonged periods of time), when not wearing appropriate PPE. Transmission can also occur from people who are infected and remain asymptomatic, but the extent to which this occurs is not fully understood and requires further research as an urgent priority. The role and extent of airborne transmission outside of health care facilities, and in particular in close settings with poor ventilation, also requires further study.
University of Maryland aerobiologist Dr. Donald Milton, lead author of the open letter the 239 scientists published, told NPR that he has “mixed feelings” about the WHO brief. “I’m glad to see they’ve moved a little bit,” he said. “I’m disappointed they didn’t move further.”