COVID-19 Surface Transmission Risk: CDC Guidance

The primary way people become infected with SARS-CoV-2 (the virus that causes COVID-19) is through exposure to respiratory droplets that carry infectious viruses. It is possible for people to become infected through contact with contaminated surfaces or objects (fomites), but the risk is generally considered low .

Background

SARS-CoV-2, the virus that causes COVID-19, is an enveloped virus, meaning its genetic material is packaged within an outer layer (envelope) of proteins and lipids. The envelope contains structures (spike proteins) to adhere to human cells during infection. The envelope for SARS-CoV-2, as with other enveloped respiratory viruses, is labile and can degrade rapidly upon contact with surfactants contained in cleaning agents and under environmental conditions.

The risk of fomite-mediated transmission depends on:

• The prevalence rate of infection in the community.
   
• The amount of virus shed by infected people (which can be substantially reduced if they wear masks).
   
• The deposition of expelled virus particles on surfaces (fomites), which is affected by airflow and ventilation.
   
• Interaction with environmental factors (e.g., heat and evaporation) that cause damage to virus particles while airborne and on fomites.
   
• The time between the time a surface becomes contaminated and the time a person touches the surface.
   
• The efficiency of the transfer of virus particles from fomite surfaces to hands and from hands to mucous membranes of the face (nose, mouth, eyes).
   
• The dose of virus necessary to cause infection through the mucous membrane route.

Due to the many factors that affect the efficiency of environmental transmission, the relative risk of fomite transmission of SARS-CoV-2 is considered low compared to direct contact, droplet transmission, or airborne transmission. However, it is unclear 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.

Infections can often be attributed to multiple routes of transmission. Fomite transmission is difficult to definitively prove, in part because respiratory transmission from asymptomatic people cannot be ruled out. Case reports indicate that SARS-CoV-2 is transmitted between people by touching surfaces that a sick person has recently coughed or sneezed on. and then directly touching your mouth, nose or eyes.

Hand hygiene is a barrier to fomite transmission and has been associated with a lower risk of infection.

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 and effectiveness of preventive measures to reduce risk. The findings from 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 , meaning that each contact with a contaminated surface has less than 1 in 10,000 chance of causing an infection.

Some studies estimated exposure risks primarily using outdoor ambient SARS-CoV-2 RNA quantification data. They noted that their QMRA estimates are subject to uncertainties that can be reduced with additional data to improve the accuracy and precision of the information input into the models. Concentrations of infectious SARS-CoV-2 on outdoor surfaces might be expected to be lower than on indoor surfaces due to dilution and air movement, as well as harsher environmental conditions such as sunlight.

A 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 disinfection of surfaces once or twice a day there was little impact on reducing the estimated risks.

Survival on surfaces

Numerous researchers have studied how long SARS-CoV-2 can survive on a variety of porous and non-porous surfaces. On porous surfaces , studies report the impossibility of detecting viable viruses within minutes or hours; On non-porous surfaces , viable viruses can be detected for days or weeks. The apparent relatively faster inactivation of SARS-CoV-2 on porous surfaces compared to non-porous surfaces could be attributed to capillary action within the pores and faster evaporation of aerosol droplets.

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. such as stainless steel, plastic and glass. 

However, experimental conditions on both porous and nonporous surfaces do not necessarily reflect real-world conditions, such as the initial amount of virus (e.g., viral load in respiratory droplets) and factors that can eliminate or degrade the virus. , such as ventilation and changing environmental conditions. They also do not take into account inefficiencies in transferring the virus between surfaces to hands and from hands to mouth, nose and eyes.

Cleaning and disinfection effectiveness

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 (for example, dirt, microbes, and other organic and chemical agents) on surfaces, but effectiveness varies depending on the type of cleaner used, the cleaning procedure, and how well the cleaning is performed. No reported studies have investigated the effectiveness of surface cleaning (with soap or detergent that does not contain a registered disinfectant external icon) in reducing SARS-CoV-2 concentrations on non-porous surfaces.

From cleaning studies focusing on other microbes, a 90-99.9% reduction in microbial levels could be possible depending on the cleaning method and surface being cleaned. In addition to the 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.

To substantially inactivate SARS-CoV-2 on surfaces, the surface must be treated with a disinfectant product external icon registered on the Environmental Protection Agency (EPA) Nexterna List External icon or technology that has been shown to be effective against the virus. Disinfectant products may also contain cleaning agents, so they are designed to clean by removing dirt and inactivating microbes. Cleaners and disinfectants should be used safely, following the manufacturer’s instructions. There has been an increase in poisonings and injuries from unsafe use of cleaners and disinfectants since the start of the COVID-19 pandemic. 

Surface disinfection has been shown to be effective in preventing secondary transmission of SARS-CoV-2 between an infected person and other people within households. However, there is little scientific support for the routine use of disinfectants in community settings, whether indoors or outdoors. to prevent the transmission of SARS-CoV-2 by fomites. In public spaces and community settings, available epidemiological data and QMRA studies indicate that the risk of SARS-CoV-2 transmission by fomites is low, compared to the risks of direct contact, droplet transmission or transmission by the air.

Routine cleaning performed effectively with soap or detergent, at least once a day, can substantially reduce virus levels on surfaces.

When targeting high-touch surfaces, cleaning with soap or detergent should be sufficient to further reduce the relatively low risk of transmission from fomites in situations where there has not been a suspected or confirmed case of COVID-19 indoors. . In situations where there has been a suspected or confirmed case of COVID-19 indoors within the last 24 hours, the presence of infectious viruses on surfaces is more likely and therefore high-touch surfaces should be disinfected.

Response to a case in an indoor environment

When a person with suspected or confirmed COVID-19 has been indoors, the virus can remain suspended in the air for minutes or hours.

How long the virus remains suspended and infectious depends on numerous factors, including the viral load in respiratory droplets or small particles, disturbance of air and surfaces, ventilation, temperature and humidity. Wearing masks consistently and correctly can substantially reduce the number of viruses indoors, including the amount of virus that lands on surfaces.

Based on limited epidemiological 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 cases of airborne transmission) and by using personal protective equipment (including any necessary protection for cleaning and disinfection products) to reduce the risk. Certain techniques can improve the fit and filtration effectiveness of masks.

After a person with suspected or confirmed COVID-19 has been indoors, the risk of fomite transmission from any surface is lower 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. In indoor environments, risks can be reduced by wearing masks (which reduces droplets that can settle on surfaces), routine cleaning, and consistent hand hygiene.