Fact check: PCR testing and viral genetic sequencing serve different purposes

A video advocating for an airline that vows to resist any future COVID-19 restrictions such as requirements to be vaccinated, wear a face mask, or provide a negative COVID-19 test, contains false information about PCR tests for COVID-19.

Reuters Fact Check. REUTERS

The video was shared on Facebook (here) and YouTube (here) and, between these platforms, has been viewed more than 250,000 times. The woman speaking in the video, Prof. Dr Dolores Cahill (, has previously spread false and misleading claims about COVID-19 (here and here). Cahill was not available for comment when asked by Reuters about the information presented in the video.

After talking through her ambitions to offer restriction-free international travel, Cahill goes on to talk about a secondary plan. “I’m launching a project in the last few weeks to sequence PCR tests,” she says (timestamp 14.40).

She continues: “Because this whole lockdown is based on positive PCR tests, but actually in the diagnostic world, in PCR, you would never diagnose with just a positive or a negative, you have to actually sequence what is the test measuring. There was 1,500 PCR tests were sequenced in October 2020, and all of them were influenza A and B. Not one were SARS-CoV-2.”

She later says that she intends to “sequence PCR” from people who consent in Ireland and the UK, and will challenge COVID-19 restrictions by taking “an injunction in Ireland to the high court, if the sequences come back as Influenza” (Timestamp 15.30).


Reuters could not find credible sources to back up claims that PCR tests should not be used to diagnose disease without sequencing each sample.

“PCR testing and sequencing are totally different things,” Stephanie Brickman said in an email to Reuters on behalf of the World Health Organisation, “It’s not that one is more accurate than the other. PCR tells you if the SARS-COV-2 virus is in a person’s body. Sequencing that virus would tell you more about the specific strain of the virus that the person had been infected with.

“Sequencing is not used for diagnosis. PCR tests are developed specifically to diagnose a disease, for example COVID-19. During the development process they are checked for specificity to be sure they do not pick up other diseases by accident.

“It would not bring any benefit to patients, who need a diagnosis, to sequence after PCR testing and it would waste time and resources. Sequencing is important for tracking variants of the virus but not for diagnosis of individuals.”

Dr Louise O'Connor (here), a researcher at the National University of Ireland, Galway, School of Medicine, also said by email that PCR tests could be used to diagnose diseases and explained how sequencing was related to the test: “The first step in designing any PCR test is to sequence the target (the thing you want to detect),” she wrote. “Usually you would sequence a whole bunch of viruses or bacteria to make sure that you have got the sequence information correct.”

Patterns from this sequence would then be chosen and tested against other similar organisms and “if there is a signal from anything other than the target - its back to the drawing board”, O’Connor said. “Most developers will try and ensure complete or 100% specificity.”

The Irish Health Service Executive (HSE) also said that it was “standard practice” to diagnose many viral infections with a positive PCT tests and that while it was “considered good practice to sequence a proportion of samples for surveillance purposes,” it was not necessary to sequence all samples before making a diagnosis.

A spokesperson for Public Health England previously told Reuters: “Molecular diagnostic tests, such as real-time PCR, are the gold standard methods for identifying individuals with an active viral infection, such as SARS-CoV-2 (the cause of COVID-19 disease), in their respiratory tract. These tests are rapid and produce results in real-time.

“It is important to note that detecting viral material by PCR does not indicate that the virus is fully intact and infectious, i.e. able to cause infection in other people. The isolation of infectious virus from positive individuals requires virus culture methods. These methods can only be conducted in laboratories with specialist containment facilities and are time consuming and complex.”


Cahill’s theory that PCR tests for COVID-19 might be inaccurate and are showing positive results for influenza is not supported by evidence (studies here and here, a previous Reuters Fact check on this claim here).

O’Connor, the HSE, the WHO, and the European Centre for Disease Prevention and Control (ECDC), all told Reuters by email that they were not aware of what Cahill’s assertion about 1,500 PCR tests being sequenced for influenza in October could refer to and indicated that such an event would be incredibly unlikely.

The HSE told Reuters: “1500 PCR tests were not sequenced in October. Despite a significant amount of testing, no influenza has been detected to date in Ireland in the current winter season” and “there is no evidence of cross-reactivity with the influenza test.”

Alexandru Niculae emailed Reuters on behalf of the ECDC to say that there could be “occasions of misclassification, mainly due to laboratory or manufacturing errors,” but that one of their experts in applied molecular epidemiology described this as “extremely unlikely.”

Brickman wrote on behalf of the WHO to say that that problems with PCR tests were “rare” and said “any widespread accuracy problems would be swiftly detected”.

O’Connor told Reuters that the sequences for influenza viruses and the virus that causes COVID-19 were different enough to ensure that tests are specific for a particular disease. She wrote: “I have looked at the performance data for most of the PCRs on the market - it varies as to what different companies have included in their specificity but in nearly all cases influenza has been checked with no cross reaction. So no, you do not have to sequence every PCR test otherwise no one would use PCR and would just go straight to sequencing.”

A claim that 1,500 COVID-19 samples were tested in Southern California, showing no SARS-CoV-2 but instead influenza, has previously circulated on social media. USA Today fact-checkers showed this claim to be baseless (here).


False. Genetic sequencing is not necessary to diagnose COVID-19, but to track strains of it. There is no evidence 1,500 PCR tests were sequenced and showed influenza was mistaken for SARS-Cov-2. PCR tests to detect COVID-19 have been developed to ensure they do not give positive results for other viruses such as influenza viruses, which are genetically distinct.

This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here