The Covid-19 virus has been isolated many times

11 November 2020
What was claimed

The Covid-19 virus has not been isolated as per Koch’s postulates, and this means PCR tests do not work.

Our verdict

This is untrue. Covid-19 has been isolated many times. It does not need to adhere to Koch’s postulates as it is a virus, and Koch’s rules have been widely disregarded now. Covid-19 PCR tests are generally very accurate.

We’ve been asked to look into claims that the Covid-19 virus hasn’t been isolated, doesn’t fulfil “Koch’s postulates”, and that this means PCR diagnosis tests aren’t working.

Isolating a virus

Firstly it is incorrect to say that the virus that causes Covid-19 has not been isolated. 

Isolating a virus means taking a pure sample of a virus from an infected being so it can be studied. There are numerous reports of the virus being isolated by teams around the world.

“SARS-CoV2 has been sampled millions of times over from infected people, including those originally found to be infected in China,” Dr Stephen Griffin, a virologist and Associate Professor at Leeds Institute of Medical Research, told Full Fact.

Koch’s postulates

It’s also incorrect to say that the virus that causes Covid-19 would need to meet Koch’s postulates, primarily because Koch’s postulates weren’t written for viruses.

Koch’s postulates were a set of rules outlined by scientist Robert Koch in 1890 to decide whether a bacteria causes a disease. The original four criteria are:

“1. The microorganism must be found in the diseased animal, and not found in healthy animals.

“2. The microorganism must be extracted and isolated from the diseased animal and subsequently grown in culture.

“3. The microorganism must cause disease when introduced to a healthy experimental animal.

“4. The microorganism must be extracted from the diseased experimental animal and demonstrated to be the same microorganism that was originally isolated from the first diseased animal.”

As noted by many, these criteria were written before the discovery of viruses, so fail to include them in their consideration of what a disease is.

Dr Griffin outlined other shortcomings in Koch’s rules. 

“The first postulate in particular is void as even at Koch’s time (which he later admitted)” he told us. “Folks knew that you could catch e.g. Cholera without necessarily becoming unwell - asymptomatic infection is a massive issue for these ideas (first and third postulates), as is the arrival of molecular biology!”

Another of Koch’s postulates was that bacteria must be able to be isolated from the host. Viruses, unlike bacteria, require host cells in which to replicate, so also cannot be isolated in the same way Koch defined with bacteria, which according to Dr Griffin, required  “culture as in a flask of media, so viruses don’t fit this idea.”

So simply put, Koch’s postulates are not a good measure of what causes disease in 2020.

PCR tests

We also know that PCR tests are identifying the virus that causes Covid-19 and working well.

As we have written before, PCR tests used in Covid-19 testing are extremely sensitive and specific at detecting Covid-19 and do not confuse other coronaviruses, such as ones that cause the common cold, for Covid-19.

“The PCR test is incredibly stringent and specific, requires two separate reactions to be positive ... and EVERY test is also sequenced, so we know it is SARS-CoV2 and not another virus,” said Dr Griffin.

So in summary, Covid-19 has been isolated, Koch’s postulates don’t have to universally apply for something to cause disease, and tests to identify the Covid-19 virus do work.

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