WHO guidance says airborne spread of Covid-19 may occur in indoor settings but calls for more evidence
Airborne transmission of Covid-19 in indoor settings might be possible, according to a new scientific brief issued by the World Health Organization (WHO). The new guidelines acknowledge some reports of airborne transmission of the virus that causes Covid-19 but stops short of confirming that the virus spreads through the air.
The WHO says “to the best of our understanding,” the virus is primarily spread through contact and respiratory droplets. Under some circumstances, airborne transmission may occur -- such as when aerosol-generating procedures are conducted in healthcare settings or potentially, in indoor crowded poorly ventilated settings elsewhere, it says. The brief explains that there have been reported outbreaks of Covid-19 in some closed settings, such as restaurants, nightclubs, places of worship or places of work where people maybe shouting, talking, or singing. In these outbreaks, aerosol transmission, particularly in these indoor locations where there are crowded and inadequately ventilated spaces where infected persons spend long periods with others, cannot be ruled out, says the WHO. The agency, however, says more studies are urgently needed to investigate such instances and assess their significance for transmission of Covid-19.
“Outside of medical facilities, 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 fitness classes. In these events, short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out,” says the WHO. However, detailed investigations of these clusters suggest that droplet and fomite transmission could also explain human-to-human transmission within these clusters, it adds. “The close contact environments of these clusters may have facilitated transmission from a small number of cases to many other people (for example, a super spreading event), especially if hand hygiene was not performed and masks were not used when physical distancing was not maintained. The role and extent of airborne transmission outside of healthcare facilities, and in particular in close settings with poor ventilation, requires further study,” says the agency.
The WHO defines airborne transmission 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.”
The new brief was issued after over 200 scientists called for greater acknowledgment of the role of airborne transmission of Covid-19 and the subsequent need for preventive measures against this risk. It provides an overview of the modes of transmission of Covid-19, what is known about when infected people transmit the virus, and the implications for infection prevention and control precautions within and outside health facilities. The experts acknowledge many unanswered questions about transmission of Covid-19 remain and research seeking to answer those questions is ongoing and is encouraged. The guidance says transmission of Covid-19 can occur through direct, indirect, or close contact with infected people through infected secretions such as saliva and respiratory secretions or their respiratory droplets, which are expelled when an infected person coughs, sneezes, talks or sings.
The report also looks at fomite transmission. It says respiratory secretions or droplets expelled by infected individuals can contaminate surfaces and objects, creating fomites (contaminated surfaces). Viable SARS-CoV-2 virus and/or RNA can be found on those surfaces for periods ranging from hours to days. Therefore, the transmission may also occur indirectly through touching surfaces in the immediate environment or objects contaminated with virus from an infected person (such as a stethoscope or thermometer), followed by touching the mouth, nose, or eyes, says the WHO. The agency says 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 advisory recommends practicing frequent hand hygiene, and physical distancing from others when possible. It suggests avoiding crowded places, close-contact settings and confined and enclosed spaces with poor ventilation, wearing fabric masks when in closed, overcrowded spaces to protect others, and ensuring good environmental ventilation in all closed settings and appropriate environmental cleaning and disinfection. The agency also calls for continuous use of a medical mask by health workers and caregivers working in all clinical areas, during all routine activities throughout the entire shift. “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,” it advises.
Besides the different transmission routes, the WHO and its partners are working to gain a better understanding about the dose of virus required for transmission to occur, the characteristics of people and situations that facilitate superspreading events such as those observed in some closed settings, the proportion of infected people who remain asymptomatic throughout the course of their infection, the proportion of truly asymptomatic persons who transmit the virus to others, and the specific factors driving asymptomatic and presymptomatic transmission.
WHO launches independent review of global Covid-19 response
Meanwhile, Dr Tedros Adhanom Ghebreyesus, WHO’s Director-General, has announced the initiation of the Independent Panel for Pandemic Preparedness and Response (IPPR) to evaluate the world’s response to the Covid-19 pandemic. The panel will be co-chaired by former Prime Minister of New Zealand Helen Clark and former President of Liberia Ellen Johnson Sirleaf. Clark had led the United Nations Development Programme from 2009 to 2017 and Sirleaf is a recipient of the Nobel Peace Prize. Operating independently, they will choose other panel members as well as members of an independent secretariat to provide support.
In November, the panel will present an interim report at the resumption of the World Health Assembly. The panel will present its substantive report in May 2021 at the World Health Assembly. “This is a time for self-reflection, to look at the world we live in and to find ways to strengthen our collaboration as we work together to save lives and bring this pandemic under control. The magnitude of this pandemic, which has touched virtually everyone in the world, clearly deserves a commensurate evaluation,” says Dr Ghebreyesus.