The probability of contagion is 0.01%: how does Covid act on surfaces and how necessary is disinfection?

The pandemic of Covid-19 declared by the World Health Organization (WHO) as a emergency of international concern on January 30, 2020. Since then, two years later, there are already millions of infections around the world.

Despite this, some countries are neglecting various health measures. Some eliminating them completely, while others give freedoms with certain limitations.

At the beginning of the pandemic, one of the most common practices adopted to prevent contagion, in addition to use of masks and hand washingwas the surface disinfection, due to the viability of the virus in these media. However, people have slowly forgotten this measure.

Currently, it has been found that the risk of fomite infection –any inanimate object that can be contaminated by a virus and transport it– It is low and that contagion is most likely through respiratory droplet exposure that carry the virus, according to the Centers for Disease Control and Prevention (CDC).

SANTIAGO (7239675)
Woman wears a mask on the Santiago Metro.

So how many are chances of getting infected with SARS-CoV-2 by touching a contaminated surface? According to Intramed, this probability is reduced to 1 in 10,000, that is, a 0.01%.

The infectologist and academic from the University of Santiago, Ignatius Silva affirms that at the beginning of the pandemic it was taken disinfection as a precautionary measurebut that “to date it has been seen that This route of infection is very ineffective.”. In addition, the hygiene campaign, as hand washing and the use of alcohol gel“it has also greatly decreased this probability.”

Recent research highlights the mucosal role to stop the spread of the virus by fomites, relating the same mechanism used by Covid-19 to spread by air, but in this case, before different surfaces.

The study used cow spit –which is similar to human saliva– and another type of virus, the human coronavirus OC43, which would have a structure similar to Covid-19, according to the researchers. OC43 can infect both people and cattle. In the human population it is considered endemic and causes mild respiratory tract infections, although it can also cause serious complications.

To generate the fomites, the virus was diluted and administered in complete media or solutions supplemented with mucins –one of the main salivary proteins and part of the mucosal barrier– in concentrations that would represent its natural presence in saliva and mucosa: between 0.1% and 5%.

They applied these samples in various surfaces and they waited different lapses of time until these created drops evaporated. They then underwent Cell cultures to see if the virus infected the samples and waited five days.

The result? The virus was almost 96% less likely to infect to an individual by direct contact of fomites, because of the mucin at 2.5%. In a concentration of 5% the possibilities were further reduced, presenting 99.9%. While samples without mucin remained highly infectious.

Thus, it was found that bovine mucins could inhibit infection of living cellsin addition to having an important and complex role in the defense of the hosts.

the infectologist Ignatius Silva relates to mucosa –from the mouth, eyes or nose– such as a gateway for the virus in case someone with SARS-CoV-2 expels these viral particles.

SARS-CoV-2 (yellow) emerging from the surface of cells. Source: NIAID-RML, AP.

Other studies have shown that the virus can stay on tables, glass, metal, or plastic for a long time. However, the researchers point out thatthe role of mucins and their molecular structure have not been well characterized in coronavirus transmission studies. Laboratory studies predicting high rates of fomite transmission have not translated into real world infectionsand mucins may be one of the culprits.”

The coronavirus has proteins called spike proteins, which are what allow them to adhere to host cells. The mutations in these proteins they are the cause that variants such as Ómicron are more transmissible.

In the study involving OC43, the bovine coronavirus, it was shown that these proteins “filled up” after a few minutes by the presence of these other proteins present in the mucosa. This union would be stimulated while the surrounding liquid is lost and the drops begin to dry.

Whereas, when the virus is in the air, it is still moist and still has free proteins that can bind to healthy cells.

According to these results, disinfecting surfaces would not be a great need.

person cleaning the floor

Regarding Covid-19, the infectologist at the Santa María Clinic, Claudia Cortesaffirms that “the measures to prevent the pandemic coronavirus, which is important and what must be emphasized, is the use of a mask and the ventilation of spaces. This is a virus that is transmitted mainly by airway and not by contact“, Thus there is no point in disinfectingfor example, the soles of shoes.

Likewise, Cortés highlights that it is difficult to extrapolate this study to humans or other coronaviruses as is SARS-CoV-2.

While Silva states that “the issue of surface disinfection should be common, not just because of Covid, due to different respiratory and gastrointestinal infections that can be spread through contaminated surfaces. So it’s good practice to keep (…), but don’t It has a great impact on the transmission of Covid”.

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