The risk of contagion of Covid-19 from touching contaminated surfaces is 1 in 10,000, according to the CDC

C. Garrido

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We have been disinfecting surfaces for more than a year as part of the protocol to avoid contagion of Covid-19, but, as the pandemic progresses, there is increasing evidence that the main route of contagion is not the contaminated objects (fomites). The US Centers for Disease Control and Prevention (CDC) released a briefing note this week stating that the main way in which people are infected with SARS-CoV-2, the virus that causes Covid-19, is through the exposure to respiratory droplets that carry infectious viruses. “It is possible that people become infected by contact with contaminated surfaces or objects (fomites), but the risk is generally considered to be low,” they point out. AND put that risk at 1 in 10,000.

After reviewing the evidence available so far, the CDC concludes that people can become infected with SARS-CoV-2 through contact with surfaces, but this is not the main route through which the new coronavirus spreads, and the risk is considered to be low. “The main way that people become infected with SARS-CoV-2 is through exposure to respiratory droplets that carry infectious viruses, “they say.

Furthermore, they suggest that, in most situations, cleaning surfaces with soap or detergent is enough to reduce the risk. They only recommend disinfection in closed community settings where a case of Covid-19 has been suspected or confirmed in the last 24 hours.

The risk of fomite transmission can be reduced «wearing masks consistently and correctly, practicing hand hygiene, cleaning and taking other measures to keep the facilities healthy, “they point out from the CDC.

The relative risk of contagion by this route is considered low, compared to direct contact, transmission by droplets or transmission through the air, due to the many factors that affect the efficiency of transmission by contact through fomites (quantity virus that is deposited on an object, interaction with environmental factors such as heat and evaporation, or the time between a surface becomes contaminated and a person touches the surface),

It is unclear what proportion of SARS-CoV-2 infections are acquired through surfaces, as there have been few reports of Covid-19 cases potentially attributed to the transmission of fomites. It is difficult to prove, in part because respiratory transmission from asymptomatic people cannot be ruled out. But case reports indicate that SARS-CoV-2 is transmitted when you touch surfaces on which a sick patient has recently coughed or sneezed, and then you bring that hand directly to your mouth, nose, or eyes. This is why hand washing is so important.

Less than 1 in 10,000

Quantitative Microbial Risk Assessment (QMRA) studies have been conducted to understand and characterize the relative risk of SARS-CoV-2 transmission through fomites. The results of these studies, noted from the CDC, suggest that the risk of SARS-CoV-2 infection through this 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. Some studies estimated exposure risks primarily using outdoor environmental SARS-CoV-2 RNA quantification data.

It is expected that Infectious SARS-CoV-2 concentrations on exterior surfaces are lower than on interiors due to harsher environmental conditions such as wind or sunlight. A QMRA study also evaluated the effectiveness of prevention measures that reduce the risk of transmission of fomites and found that hand hygiene could substantially reduce the risk of transmission of SARS-CoV-2 from contaminated surfaces, while with the Disinfection of surfaces once or twice a day had little impact in reducing the estimated risks.

On the survival time of the virus on surfaces, the CDC recalls that on porous surfaces, studies detail the impossibility of detecting viable viruses in minutes or hours, while on non-porous surfaces, viable viruses can be detected for days or weeks.

Data from surface survival studies indicate that a 99% reduction in SARS-CoV-2 infectious and other coronaviruses under typical indoor environmental conditions in 3 days on common non-porous surfaces such as stainless steel, plastic and glass. However, they caution, experimental conditions on porous and non-porous surfaces do not necessarily reflect real-world conditions, such as the initial amount of virus and factors that can eliminate or degrade it, such as ventilation and changing environmental conditions. They also do not take into account the inefficiency in transferring the virus from surfaces to hands and from hands to mouth, nose, and eyes. When surface survival data and real-world transmission factors are taken into account, the risk of fomite transmission after a person with Covid-19 has been indoors is lower after 3 days (72 hours), regardless of when it was last cleaned.

Cleaning with soap and water

Both cleaning with soap or detergent and disinfection with a product designed to inactivate SARS-CoV-2 can reduce the risk of fomite transmission. There are no studies that have investigated the effectiveness of surface cleaning (with soap or detergent that does not contain a registered disinfectant) to reduce SARS-CoV-2 concentrations on non-porous surfaces. But, from cleanup studies focused on other microbes, a 90-99.9% reduction in microbial levels depending on cleaning method and the surface to be cleaned. In addition to the physical removal of SARS-CoV-2 and other microbes, the CDC expects surface cleaning to degrade the virus, as surfactants in cleaners can alter and damage the membrane of an enveloped virus such as SARS- CoV-2.

But if the aim is to substantially inactivate SARS-CoV-2 on surfaces, the area should be treated with a disinfectant product that has been shown to be effective against the virus. These products should always be used with caution and following the manufacturer’s instructions. The CDC recalls that some types of disinfection applications, particularly those that include fogging, are not safe or effective in inactivating the virus if not used correctly.

Surface disinfection has been shown to be effective in preventing secondary transmission of SARS-CoV-2 between an infected person and others within homes. However, the CDC notes that there is little scientific support for the routine use of disinfectants in community settings, whether indoors or out.

In public spaces and community settings, available epidemiological data and QMRA studies indicate that the risk of transmission of SARS-CoV-2 from fomites is low, compared to the risks of direct contact, droplet transmission, or airborne transmission.

For the CDC, a routine cleaning thoroughly with soap or detergent, at least once a day, is sufficient to substantially reduce virus levels on surfaces. They only advise disinfecting in situations where there has been a suspected or confirmed case of Covid-19 indoors in the last 24 hours, as the presence of infectious viruses on surfaces is more likely.

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