The risk of contagion by contact with an object is less than 0.04%
The remains of the Covid-19 virus that may be left on hard surfaces such as doorknobs, traffic light buttons, ATMs or gas pumps are rarely contagious, according to an international study led by Tufts University of Massachusetts.
The study, carried out in the Boston metropolitan area, shows that there are usually remains of the SARS-CoV-2 coronavirus in public places when there is a high incidence of Covid-19 in the community. But it is rare for the virus to be viable and in sufficient quantity to cause an infection to anyone who touches a contaminated surface.
The new data is part of the scientific debate on the role of fomites – any inanimate object from which contagion can occur.
Although in the first months of the pandemic people had assumed, due to health authorities’ warnings that contagion was imminent after touching contaminated surfaces, this view has been questioned as it has been seen that the Covid-19 virus is not transmitted in the same way as the flu.
The investigation has been based on searching for the coronavirus in twelve public places in the city of Sommerville between March and June.
According to the results presented on November 1 on the medRxiv server, 8.3% of the samples analyzed had virus remnants detectable by PCR.
The places where viral material was most frequently found were a garbage container, where 25% of the samples were positive, and the door of a liquor store, where 15% were also positive.
However, only one in ten positive samples had enough virus debris to be able to quantify it. The other nine tested positive but were below the measurement limit.
In those that could be quantified, all of them coming from shop doors, it was estimated that the risk of contagion from touching the contaminated surface was 4 in 10,000, or 0.04%, for the one with the most virus remains. For the one with the least, the risk dropped to 1 in 100,000, or 0.001%.
“The low risk of infection estimated in this study reinforces that priority is given to reducing infections by aerosols and drops and by close contacts,” conclude the authors of the research from Tufts in Massachusetts, Berkeley in California, Basel in Switzerland and Imperial College in London.
Previous studies have shown that coronaviruses can linger from several hours to several days on plastic or metal surfaces. But these studies have been based on depositing large amounts of virus on surfaces and waiting for them to degrade.
“None of them present scenarios equivalent to real-life situations,” warned microbiologist Emanuel Goldman, from Rutgers University in New Jersey (USA), in an article published in August in The Lancet Infectious Diseases.
According to Goldman, “the possibility of transmission through inanimate surfaces is very small, and only in situations where an infected person coughs or sneezes on the surface, and another person touches it shortly after, within a maximum period of 1-2 hours”.
The authors of the new research defend in medRxiv that “the disinfection of surfaces that are frequently touched is probably useful to prevent possible cases of transmission by fomites; disinfecting hands after touching public surfaces can reduce the risk of transmission even more ”. But they emphasize that the probability of contagion of SARS-CoV-2 by touching contaminated surfaces is lower than for other respiratory pathogens.