Covid-19: Can the viruses be spread indirectly via the air?

Harvey Fineberg is a respectable man. The American doctor held key positions at Harvard University and was president of the Institute of Medicine, now known as the National Academy of Medicine. Fineberg specializes in health issues and advises the U.S. government on the Standing Committee on Emerging Infectious Diseases when it comes to making public health policy decisions. Fineberg is also the author of The Swine Flue Affair, which takes a critical look at a vaccination campaign in 1976 that aimed to vaccinate 40 million Americans against a new variant of Spanish flu – but the population was so poorly informed was that practically nobody went to vaccinate.

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Corona epidemic study

So when Harvey Fineberg reports on the Corona crisis, you should take it seriously. In an open letter from the National Academics of Science, Engineering and Medicine, the physician, along with other scientists, wrote that “a possible aerosolization of the virus during normal breathing” appears possible. Translated into colloquial language, this means that exhaling could transmit Sars-CoV-2.

So far there has been a lot of warning about sneezing and coughing, including wet pronunciation. But can viruses actually be transmitted indirectly, i.e. only via the air?

The question of possible routes of transmission of a virus is extremely relevant for epidemiologists. It is necessary to clarify them in order to make reliable forecasts of the number of infected people. The AIDS virus, for example, is almost exclusively transmitted through blood, sperm, vaginal secretions and breast milk, which is why HIV-infected people are hardly infectious in everyday life. Once the route of infection was clarified, it was clear how to protect yourself from infection.

And the more alarming is the assumption that Sars-CoV-2 can be transmitted through the breathing air.

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Corona virus - test evaluation in the laboratory

Criticism from scientists

Harvey Fineberg refers to the ongoing research on contagiosity, i.e. the transferability of the virus. Scientists at various research institutes are infecting ferrets, mice, cats and dogs with the new corona virus – and are monitoring the path of the virus in their bodies, their health and also the transmission to other animals.

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Ferrets particularly well suited for testing

Ferrets in particular seem to be particularly well suited for experiments. Researchers from the Friedrich Löffler Institute for Animal Health on the island of Riems in Mecklenburg-Western Pomerania, among others, were able to show that the virus behaves in the body of small animals very similarly to that in humans: Sars-Cov-2 reproduces there especially in the upper respiratory tract, some animals develop a fever two to eight days after the infection, and serious illnesses do not occur. The animals are currently considered the best model by scientists to investigate portability, medication and vaccines.

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Corona virus - test evaluation in the laboratory

Criticism from scientists

South Korean researchers from Cheongju have devoted themselves precisely to the question that Harvey Fineberg is addressing: They have tried ferrets to clarify the transferability and have now published their results in the journal “Cell Host & Microbe”.

In the laboratory in Cheongju, non-infected ferrets with infected ferrets were placed in the same cage. After two days, the virus was found in all animals. The animals developed a slight fever, coughed between the second and sixth day and showed reduced physical activity.

ferret (Mustela putorius furo) England (ferret

Scientists have high hopes for ferrets when researching coronaviruses

Credit: Getty Images / Westend61

Ferrets kept in a separate cage nearby, into which only the ambient air of the infected ferret was introduced through an air stream, did not develop a fever, cough, or other symptoms.

However, and this is important: RNA of the virus could be detected in the feces and in the nasal mucosa of the animals in the removed cages. In a ferret, the researchers found not only the virus, but even antibodies against it – this means that the immune system was already activated, so the pathogen in the body had been classified as a potential hazard.

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The conclusion of the South Korean scientists is nevertheless: “Continuous direct contact is required” for a relevant conference. However, transmission through the air cannot be completely ruled out.

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So are the warnings from Harvey Fineberg and the other authors justified? After all, they themselves point out that the detection of the viruses with ferrets kept away was verified by a polymerase chain reaction, a PCR: With this method, extremely small amounts of virus particles can be detected. However, this very high sensitivity has one disadvantage: it may also detect amounts of virus that are too small to trigger an illness.

Transferable but not infectious

In the letter, the authors therefore formulate: Studies on SARS-CoV-2 that rely on PCR detection to detect the presence of viral RNA “may not have found a sufficient number of viable viruses that can trigger an infection.”

A virus can be easily transmitted, that is, contagious. Another question that needs to be clarified is whether it also makes you sick, i.e. infectious. Thomas Mettenleiter, President of the Friedrich Löffler Institute on the island of Riems, agrees. “For Sars-CoV-2, there has been no correlation between genome detection (qPCR data) and virus isolation data, which allow a reliable statement here. Further investigations are necessary here that correlate such data, ”he says. “However, it is usually the case that detection by PCR is still possible at very low genome loads, but often no more infectiousness can be detected.”

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Course of a corona infection

Roman Wölfel, senior physician and head of the Bundeswehr Institute for Microbiology in Munich, also believes that the previous results on portability and infectivity should not be overinterpreted. “For the animals of the indirect contact model, the data from the studies described here do not convince me of aerosol transmission.”

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Ferrets as animal models are considered to be very well established – so that one can conclude from them the course of an infection in humans. According to Wölfel, if an infection had actually been transmitted, a clear reaction of the immune system would have to be expected. The detection of a low concentration of antibodies in only one animal does not convince him. “The hypothesis of an indirect transmission of Sars-CoV-2 still requires further investigation and should not be considered as reliable based on this study.”

“It’s voluntary, I don’t think I want to do it”

The United States has killed more than 1,400 coronaviruses within 24 hours. President Trump announces a new directive that people should now wear masks. But he also makes it clear who doesn’t want to stick to it.

Nobody can currently wait until questions about transferability and infectiousness have been finally clarified scientifically. Precautions must be taken, especially in clinics with Covid 19 patients. If the virus turned out to be indirect, airborne, the protective measures would have to be increased significantly.

“A proven motto in medicine is that proactive action is often better than waiting too hesitantly for conclusive scientific evidence,” says Clemens Wendtner, chief physician of infectious diseases and tropical medicine and head of the special unit for highly contagious life-threatening infections at the Schwabing Clinic in Munich . “Patient and employee protection are the greatest asset.” Doctors and nurses in intensive care units should therefore take special protective measures. You should “work with a full protective suit, face shield and FFP3 masks”.

And Harvey Fineberg and the open letter? The letter is a warning that Sars-Cov-2 is still under-researched. And it bears witness to the fact that one should continue to act with extreme caution in the developing epidemic.


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