This week, two entities that are authorities on combatting COVID-19 in facilities released updated guidance. The ASHRAE Epidemic Task Force updated its airborne transmission guidance on the virus. And, CDC provided an update on surface transmission for community environments. Here are summaries of both statements:
ASHRAE Update On Airborne Transmission
On April 5, 2021, the ASHRAE Epidemic Task Force released its updated statement on the airborne transmission of SARS-CoV-2 in buildings:
“Airborne transmission of SARS-CoV-2 is significant and should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures.”
It replaces the April 2020 statement that said airborne transmission was “sufficiently likely” that airborne precautions should be taken. At that time both, the World Health Organization (WHO) and the Centers for Diseases Control (CDC), contended that transmission of SARS-CoV2 was by droplet and fomite modes, not airborne. Subsequently, both have acknowledged the risk of airborne transmission indoors.
“This may seem like a small step, but we feel it is important to leave no doubt about our position, given the muted support for ventilation and filtration as important tools in the effort to stop the pandemic, from some organizations that should be leading more strongly,” said William P. Bahnfleth, Ph.D., P.E., ASHRAE Epidemic Task Force chair.
The ASHRAE Epidemic Task Force has been developing and disseminating guidance for the control of airborne transmission of SARS-CoV-2 since its formation in March 2020.
“ASHRAE volunteers have played a huge role in evaluating evidence and developing detailed guidance to improve indoor environmental quality,” said Bahnfleth. “The public, globally, is benefitting from the volunteer efforts of some of the most knowledgeable scientists and engineers in our field and this updated guidance is proof of it.”
CDC Update On Surface (Fomite) Transmission
Also on April 5, 2021, CDC released its Science Brief: SARS-CoV-2 and Surface (Fomite) Transmission for Indoor Community Environments. In the summary heading of this brief, CDC states:
“The principal mode by which people are infected with SARS-CoV-2 (the virus that causes COVID-19) is through exposure to respiratory droplets carrying infectious virus. It is possible for people to be infected through contact with contaminated surfaces or objects (fomites), but the risk is generally considered to be low.”
Cleaning and, separately, disinfection are still considered impactful and important, and the statement from CDC begins:
Background: SARS-CoV-2, the virus that causes COVID-19, is an enveloped virus, meaning that its genetic material is packed inside an outer layer (envelope) of proteins and lipids. The envelope contains structures (spike proteins) for attaching to human cells during infection. The envelope for SARS-CoV-2, as with other enveloped respiratory viruses, is labile and can degrade quickly upon contact with surfactants contained in cleaning agents and under environmental conditions. The risk of fomite-mediated transmission is dependent on:
- The infection prevalence rate in the community
- The amount of virus infected people expel (which can be substantially reduced by wearing masks)
- The deposition of expelled virus particles onto surfaces (fomites), which is affected by air flow and ventilation
- The interaction with environmental factors (e.g., heat and evaporation) causing damage to virus particles while airborne and on fomites
- The time between when a surface becomes contaminated and when a person touches the surface
- The efficiency of transference of virus particles from fomite surfaces to hands and from hands to mucous membranes on the face (nose, mouth, eyes)
- The dose of virus needed to cause infection through the mucous membrane route
Because of the many factors affecting the efficiency of environmental transmission, the relative risk of fomite transmission of SARS-CoV-2 is considered low compared with direct contact, droplet transmission, or airborne transmission 1, 2. However, it is not clear what proportion of SARS-CoV-2 infections are acquired through surface transmission. There have been few reports of COVID-19 cases potentially attributed to fomite transmission 1, 2. Infections can often be attributed to multiple transmission pathways. Fomite transmission is difficult to prove definitively, in part because respiratory transmission from asymptomatic people cannot be ruled out 3, 4, 5. Case reports indicate that SARS-CoV-2 is transmitted between people by touching surfaces an ill person has recently coughed or sneezed on, and then directly touching the mouth, nose, or eyes 3, 4, 5. Hand hygiene is a barrier to fomite transmission and has been associated with lower risk of infection 6.
Quantitative microbial risk assessment (QMRA) studies have been conducted to understand and characterize the relative risk of SARS-CoV-2 fomite transmission and evaluate the need for and effectiveness of prevention measures to reduce risk. Findings of these studies suggest that the risk of SARS-CoV-2 infection via the fomite transmission route is low, and generally less than 1 in 10,000, which means that each contact with a contaminated surface has less than a 1 in 10,000 chance of causing an infection 7, 8, 9. Some studies estimated exposure risks primarily using outdoor environmental SARS-CoV-2 RNA quantification data. They noted that their QMRA estimates are subject to uncertainty that can be reduced with additional data to improve the accuracy and precision of information that is entered into the models. Concentrations of infectious SARS-CoV-2 on outdoor surfaces could be expected to be lower than indoor surfaces because of air dilution and movement, as well as harsher environmental conditions, such as sunlight. One QMRA study also evaluated the effectiveness of prevention measures that reduce the risk of fomite transmission and found that hand hygiene could substantially reduce the risk of SARS-CoV-2 transmission from contaminated surfaces, while surface disinfection once- or twice-per-day had little impact on reducing estimated risks 9.
The CDC Brief then addresses Surface Survival, Effectiveness of Cleaning and Disinfection, and Response to a Case in an Indoor Environment. (Visit this link to view entire Brief, including these sections.)
The CDC concludes its April 5 update, stating: People can be infected with SARS-CoV-2 through contact with surfaces. However, based on available epidemiological data and studies of environmental transmission factors, surface transmission is not the main route by which SARS-CoV-2 spreads, and the risk is considered to be low. The principal mode by which people are infected with SARS-CoV-2 is through exposure to respiratory droplets carrying infectious virus. In most situations, cleaning surfaces using soap or detergent, and not disinfecting, is enough to reduce risk. Disinfection is recommended in indoor community settings where there has been a suspected or confirmed case of COVID-19 within the last 24 hours. The risk of fomite transmission can be reduced by wearing masks consistently and correctly, practicing hand hygiene, cleaning, and taking other measures to maintain healthy facilities.
Visit the CDC website here to read the entire April 5 statement, “Science Brief: SARS-CoV-2 and Surface (Fomite) Transmission for Indoor Community Environments.”
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