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Undocumented infections by SARS-CoV-2 and indirect transmission are two key features of Covid-19 pandemic

In the last article I updated you about the current most promising treatments against Covid-19 pandemic. In this one, I want to focus on two crucial features of the pandemic: the importance of undocumented infections and the indirect transmission of the virus.

Undocumented infections of SARS-CoV-2

A study published on Science reveals that undocumented infections were responsible for 79% of documented cases in China before quarantine began. This shocking data is an explanation for the rapid spreading of the virus all over the world, and also makes illogical the strategy of some countries (US, UK and Sweden among others) to limit the testing. The percentage was elaborated through a complex model established by Li and colleagues. They simulated the spatiotemporal infection spreading in 375 Chinese cities. Results indicate that, between 10th and 23rd of January 2020, 86% of infections were undocumented. Therefore, most (79%) Covid-19 diagnosed patients were infected by undocumented cases, until travel ban and strict control measures were taken into action. It is conceivable that undocumented Covid-19 cases generally present mild symptoms, but they contribute greatly to the pandemic spreading.

This model was elaborated on a Chinese region with no restrictions or control; therefore percentages might likely change in other countries, depending on the viral surveillance, testing and control activity. Moreover, it is not possible to conclude that testing every potential case would allow to eliminate the pandemic, but for sure it would significantly slow down its spreading. 

Indirect transmission of SARS-CoV-2

In this section I address few questions about survival of the virus outside the human body, and therefore the indirect transmission of the infection. A recent study published by van Doremalen and colleagues revealed that SARS-CoV-2 has different lifespan on different surfaces. They put the virus on steel, copper, plastic and cardboard and they measured how much time the virus took to be unable to infect. Infectivity was detected for 48 hours on steel and for 72 hours on plastic. It lasts less than 8 hours on copper and a bit more than 24 hours on cardboard. Halving of infectivity occurs after 2 hours on copper, 5 hours on cardboard, 6 hours on steel and 7 hours on plastic. These measures might change according to the specific type of material and external conditions (temperature, humidity), anyway they provide valid relevance about the indirect transmission of the virus. In aerosol, virus infectivity was tested for a period of 3 hours and it didn’t show relevant reduction. 

These results show that indirect infection is plausible and the only way to limit it is to clean surfaces properly (steel and plastic are easily washable with detergents), to wash hands as often as possible and to avoid touching the face. In fact, a weakness of the virus is that it’s very labile and detergents and soap are sufficient to eliminate it. 

Sources

https://science.sciencemag.org/content/early/2020/03/13/science.abb3221

https://www.medrxiv.org/content/10.1101/2020.03.09.20033217v2

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