An article that claims that researchers have not proven that SARS-CoV-2 causes COVID-19 uses outdated criteria and presents incomplete facts to make its arguments. The article focuses on how researchers have not fulfilled Koch’s postulates – a 19th-century set of criteria to show a microbe causes a disease – as they were originally understood.

Koch’s postulates were presented in 1890 (here and here), at a time when germ theory was still controversial and before the discovery of viruses, to which Koch’s postulates, as they were originally written, do not apply. Koch himself recognised the postulates had exceptions (here), and they have not been established for many disease-causing microbes. Modern criteria that confirm that a virus causes a disease have been demonstrated for COVID-19.

KOCH’S POSTULATES

The article (here) says: “Koch’s postulates are the decisive criteria for the scientific detection of a virus” then gives a list of these postulates in their original form. Postulate One: “The micro-organism must be detectable in all cases of disease with the same symptoms, but not in healthy individuals.”

This postulate was made obsolete even in Koch’s own time as asymptomatic carriers of disease-causing microbes were discovered, prompting Koch to amend or abandon this postulate (here and here and here). Postulate Two: “The microorganism can be transferred from the diseased individual to a pure culture (isolation).”

This is in general understood to mean that the microbe should be able to be grown in something a sterile growth medium. Many types disease carrying microbes, including types of bacteria, still cannot be grown like this (here and here), however are well established as the cause of diseases. Viruses cannot reproduce themselves, and so cannot be grown at all as a “pure culture” as Koch would have envisioned it, however it is well-established that viruses cause diseases (here and here and here). Postulates three and four also cannot be fulfilled as written if the pure culture of the disease is unavailable.

Postulate Three: “A previously healthy individual, after infection with the micro-organism from the pure culture, shows the same symptoms as the one from which the micro-organism originally originated.”

While purposeful infection of humans with disease has been done historically, this is rare due to ethical concerns. However, the UK clinical trials ethics body last month approved a COVID-19 challenge study, in which healthy young volunteers will be exposed to the SARS-CoV-2 virus in a safe and controlled environment (here).

In most cases, the individuals used to fulfil the third postulate tend to be animals. This makes it impossible to fulfil this postulate, and by extension, the fourth postulate, in diseases specific to humans (here). Postulate Four: “The microorganism can be transferred from the infected and diseased individuals back into a pure culture.” While not an original postulate of Koch’s (here), this is essentially a repeat of step two using a different source.

The article goes on to say: “From textbooks (e.g., White/Fenner. Medical Virology, 1986, p. 9), as well as from leading virus researchers such as Luc Montagnier or Dominic Dwyer, it has been found that particle cleaning – i.e., the separation of an object from everything that is not that object, such as the Nobel Prize winner Marie Curie in 1898 purified 100 mg of radium chloride by extraction from tons of pitchblende – is an essential prerequisite for proving the existence of a virus.” The Medical Virology textbook can be seen online (here). Page 9 appears missing, but later references to Koch’s postulates (pp. 238-244) recognise the limits of these postulates and do not imply that they must be fulfilled to establish disease causation. The book notes: “two tools that are of central importance in assisting the epidemiologist are immunologic investigations and demonstration of the presence of the viral genome in tumor cells by the use of nucleic acid probes or the polymerase chain reaction”. ISOLATION The article says that methods to isolate the virus that causes COVID-19 do not count, as they did not “purify the virus.”

Siouxsie Wiles, an associate professor at University of Auckland’s Department of Molecular Medicine and Pathology, has addressed claims like these, saying: “viruses need a host cell to replicate in” so samples of the virus will always be mixed with other genetic material (here and here), however this does not prevent the virus from being studied or genetically sequenced. Luc Montagnier, who the article references to support its argument that a particle must be isolated in order to establish causation, has specifically rejected suggestions that purification is necessary when isolating viruses (here).

DETECTING COVID-19 Virus research has been transformed by genetic techniques developed decades after Koch’s death (www.pnas.org/content/106/1/6 and here and here). These techniques have been used to identify COVID-19 (here and here and here).

Updated criteria for determining viruses that cause disease are sometimes still referred to as Koch’s postulates (here) These involve animals being infected with the suspect virus (here and here) and viruses being grown in cells (here), which would not be considered a “pure culture” as Koch would have understood it (here) and would not fulfil the purification criteria mentioned previously even though other microbes are removed (here).

VERDICT False. Koch’s postulates, as they were originally understood, do not need to be demonstrated in order to establish that a microbe causes a disease. SARS-CoV-2 causes COVID-19 and has been shown to do so by modern standards, including criteria modelled on Koch’s postulates, as well as genetic techniques that were not developed until decades after Koch’s death.

This article was produced by the Reuters Fact Check team. Read more about our work to fact-check social media posts here .

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