CDC Science Brief: SARS-CoV-2 and Surface (Fomite) Transmission

#15,898

When health authorities like the CDC, ECDC, CHP, and WHO issue health guidance during disease outbreak they don't always have the best scientific evidence on which to base them.  New, or mutated, diseases can behave differently than older ones, and it can take time - often months - to gather and analyze data.

Just last weekend, in Preprint: Low Risk of Transmission for SARS-CoV-2 by Contaminated Coins and Bank Notes, we looked at a study that reassuringly found that coins and banknotes were less apt to transfer COVID-19 than previously believed.  

While such transmision was still possible, it was deemed `unlikely' by the authors. 

In that blog, we also looked at a January Nature.com feature (see COVID-19 rarely spreads through surfaces. So why are we still deep cleaning?) that questioned the `deep cleaning' frenzy that has gripped many businesses, schools, and homes in order to prevent COVID transmission. 

• Some of this mania was initially sparked by China's high profile `fogging' of city streets, and `disinfection army' approach to controlling the virus (see WHO: Cleaning & Disinfection of Environmental Surfaces in the Context of COVID-19).  
• While some of it was employed by businesses and schools as a way to reassure customers, workers, students, and others of the safety of their offices, stores, restaurants, and classrooms. 
• And of course, manufacturers of cleaning products did whatever they could to encourage their use.

But a year ago, the CDC's COVID-19 Transmission webpage clearly stated:

The virus is thought to spread mainly from person-to-person. 

• Between people who are in close contact with one another (within about 6 feet).
• Through respiratory droplets produced when an infected person coughs, sneezes, or talks.
• These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs.
• COVID-19 may be spread by people who are not showing symptoms.

But they also granted that: 

The virus may be spread in other ways

It may be possible that a person can get COVID-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose, or possibly their eyes. This is not thought to be the main way the virus spreads, but we are still learning more about how this virus spreads.

And the CDC even warned - via an MMWR report published in April of 2020 (see MMWR: Cleaning and Disinfectant Chemical Exposures & Temporal Associations with COVID-19) - of the dangers of using harsh or toxic chemicals in environmental cleaning. 

NOTE: `Cleaning' was defined as using `soap and water', while `disinfecting' was using either a bleach & water solution or an EPA approved disinfectant. 

Around that time the CDC issued guidance on environmental cleaning for COVID (see CDC Extended Guidance for Cleaning and Disinfecting (Homes, Schools, Businesses, etc.), advising:

When to Clean and When to Disinfect

Cleaning with products containing soap or detergent reduces germs on surfaces by removing contaminants and may also weaken or damage some of the virus particles, which decreases risk of infection from surfaces.

When no people with confirmed or suspected COVID-19 are known to have been in a space, cleaning once a day is usually enough to sufficiently remove virus that may be on surfaces and help maintain a healthy facility.

Disinfecting (using U.S. Environmental Protection Agency (EPA)’s List N) kills any remaining germs on surfaces, which further reduces any risk of spreading infection.

 

You may want to either clean more frequently or choose to disinfect (in addition to cleaning) in shared spaces if certain conditions apply that can increase the risk of infection from touching surfaces:

• High transmission of COVID-19 in your community,
• Low number of people wearing masks,
• Infrequent hand hygiene, or
• The space is occupied by certain populations, such as people at increased risk for severe illness from COVID-19
• If there has been a sick person or someone who tested positive for COVID-19 in your facility within the last 24 hours, you should clean AND disinfect the space.

Yesterday the CDC published a new scientific brief on what is known about fomite transmission of COVID-19 (aka SARS-CoV-2), that reiterates much of the advice previously issued.  

Daily cleaning (as opposed to disinfecting) is generally adequate unless there has been a high risk exposure, and the risks of being infected via fomites is considered low in any case.   Obviously, if you have a COVID case in your home or office, you are going to want to disinfect and clean. 

I've reproduced the text of the report below, but you'll want to follow the link for the (extensive) list of references. 

Science Brief: SARS-CoV-2 and Surface (Fomite) Transmission for Indoor Community Environments

Updated Apr. 5, 2021
 
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.

 

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.

Surface survival

Numerous researchers have studied how long SARS-CoV-2 can survive on a variety of porous and non-porous surfaces 10, 11, 12, 13, 14, 15. On porous surfaces, studies report inability to detect viable virus within minutes to hours; on non-porous surfaces, viable virus can be detected for days to weeks. The apparent, relatively faster inactivation of SARS-CoV-2 on porous compared with non-porous surfaces might be attributable to capillary action within pores and faster aerosol droplet evaporation 16.

Data from surface survival studies indicate that a 99% reduction in infectious SARS-CoV-2 and other coronaviruses can be expected under typical indoor environmental conditions within 3 days (72 hours) on common non-porous surfaces like stainless steel, plastic, and glass 10, 11, 12, 13, 15. However, experimental conditions on both porous and non-porous surfaces do not necessarily reflect real-world conditions, such as initial virus amount (e.g., viral load in respiratory droplets) and factors that can remove or degrade the virus, such as ventilation and changing environmental conditions 8, 9. They also do not account for inefficiencies in transfer of the virus between surfaces to hands and from hands to mouth, nose, and eyes 8, 9. In fact, laboratory studies try to optimize the recovery of viruses from surfaces (e.g., purposefully swabbing the surface multiple times or soaking the contaminated surface in viral transport medium before swabbing). When accounting for both surface survival data and real-world transmission factors, the risk of fomite transmission after a person with COVID-19 has been in an indoor space is minor after 3 days (72 hours), regardless of when it was last cleaned 8, 9, 10, 11, 12, 13, 15.

Effectiveness of cleaning and disinfection

Both cleaning (use of soap or detergent) and disinfection (use of a product or process designed to inactivate SARS-CoV-2) can reduce the risk of fomite transmission. Cleaning reduces the amount of soil (e.g., dirt, microbes and other organic agents, and chemicals) on surfaces, but efficacy varies by the type of cleaner used, cleaning procedure, and how well the cleaning is performed. No reported studies have investigated the efficacy of surface cleaning (with soap or detergent not containing a registered disinfectant) for reducing concentrations of SARS-CoV-2 on non-porous surfaces. From studies of cleaning focused on other microbes, a 90–99.9% reduction of microbe levels could be possible depending on the cleaning method and the surface being cleaned 17, 18. In addition to physical removal of SARS-CoV-2 and other microbes, surface cleaning can be expected to degrade the virus. Surfactants in cleaners can disrupt and damage the membrane of an enveloped virus like SARS-CoV-2 19, 20, 21.

To substantially inactivate SARS-CoV-2 on surfaces, the surface must be treated with a disinfectant productexternal icon registered with the Environmental Protection Agency’s (EPA’s) List Nexternal icon or technology that has been shown to be effective against the virus 22. Disinfectant products might also contain cleaning agents, so they are designed to clean by both removing soil and inactivating microbes. Cleaners and disinfectants should be used safely, following the manufacturer guidance. There have been increases in poisonings and injuries from unsafe use of cleaners and disinfectants since the start of the COVID-19 pandemic 23. Some types of disinfection applications, particularly those including fogging or misting, are neither safe nor effective for inactivating the virus unless properly used 24.

Surface disinfection has been shown to be effective for preventing secondary transmission of SARS-CoV-2 between an infected person and other people within households 25. However, there is little scientific support for routine use of disinfectants in community settings, whether indoor or outdoor, to prevent SARS-CoV-2 transmission from fomites. In public spaces and community settings, available epidemiological data and QMRA studies indicate that the risk of SARS-CoV-2 transmission from fomites is low—compared with risks from direct contact, droplet transmission or airborne transmission 8, 9. Routine cleaning performed effectively with soap or detergent, at least once per day, can substantially reduce virus levels on surfaces. When focused on high-touch surfaces, cleaning with soap or detergent should be enough to further reduce the relatively low transmission risk from fomites in situations when there has not been a suspected or confirmed case of COVID-19 indoors. In situations when there has been a suspected or confirmed case of COVID-19 indoors within the last 24 hours, the presence of infectious virus on surfaces is more likely and therefore high-touch surfaces should be disinfected 26.

Response to a case in an indoor environment

When a person with suspected or confirmed COVID-19 has been indoors, virus can remain suspended in the air for minutes to hours. The length of time virus remains suspended and is infectious depends on numerous factors, including viral load in respiratory droplets or in small particles, disturbance of air and surfaces, ventilation, temperature, and humidity 27, 28, 29, 30, 31. Wearing masks consistently and correctly can substantially reduce the amount of virus indoors, including the amount of virus that lands on surfaces 32.

Based on limited epidemiologic and experimental data, the risk of infection from entering a space where a person with COVID-19 has been is low after 24 hours. During the first 24 hours, the risk can be reduced by increasing ventilation and waiting as long as possible before entering the space (at least several hours, based on documented airborne transmission cases), and using personal protective equipment (including any protection needed for the cleaning and disinfection products) to reduce risk. Certain techniques can improve the fit and filtration effectiveness of masks 32.

After a person with suspected or confirmed COVID-19 has been in an indoor space, the risk of fomite transmission from any surfaces is minor after 3 days (72 hours). Researchers have found that 99% reduction in infectious SARS-CoV-2 on non-porous surfaces can occur within 3 days 8, 9, 10, 11, 12, 13. In indoor settings, risks can be reduced by wearing masks (which reduces droplets that can be deposited on surfaces), routine cleaning, and consistent hand hygiene.

Conclusion

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.

         (Continue . . . . )

I suspect most of those who are inclined to clean and disinfect rigorously are unlikely to be swayed by this data, but it should be reassuring to those of us who don't possess a `housekeeping gene', and figure if a virus gets in, it does so at its own peril.