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FactCheck: Can a common cold cause a positive Covid-19 test?

A claim that started in the US, which suggests that a case of cold or flu, or a previous vaccine against them, can give a positive Covid-19 test result, has reached Ireland.

A CLAIM THAT a person with a cold or flu will test positive for Covid-19 has surfaced in Ireland in recent days after starting in the US.

The claim, which has appeared in several forms on social media, centres on an argument that conflates antibody testing for previous Covid-19 infections with viral testing for an existing case of Covid-19. 

The Claim

The claim originated in the US based on screenshots from the Center for Disease Control and Prevention (CDC) which said that antibody testing for Covid-19 could return a positive result if the test detected antibodies for a cold. 

The CDC statement has been used as the basis for claims on social media that tests for current Covid-19 cases will return a positive if the person tested has the cold or flu, or has ever received a vaccine for a cold or flu. There is no vaccine for a cold, despite what these claims say. 

The claim has appeared on social media in Ireland in recent days. 

One Ireland-based Facebook page that shared the claim wrote that “coronavirus testing can also be common cold or flu virus if you ever had the cold and flu jab you will test positive for covid 19”.

The post has been viewed 33,000 times in six days and attracted 183 shares. 

The claim was spread in the United States on multiple platforms, including from a website called Intellihub.com, which wrote “Shocker: CDC admits Covid-19 ‘positive result’ just means you’ve previously contracted the ‘common cold’”.

Another Facebook post in Ireland that shares a screenshot of the article’s headline has been viewed 19,000 times in six days and includes the caption: “The government is trying to crash the economy over the common cold.”

The Evidence

The statement from the CDC relates to antibody testing for past infection with Covid-19, rather than viral testing for current cases of the virus, which is a completely different thing. 

Viral tests are used to identify current cases of Covid-19 by using samples from the respiratory system. In Ireland, viral tests are carried out by swabbing the inside of the nose and the back of the throat.

Antibody tests, however, check the blood of the person being tested for antibodies that fight against Covid-19, which can indicate that a person has previously had a case of Covid-19 which their body sought to protect itself against by producing those antibodies.

It can take one to three weeks for a body to make antibodies after infection.

Viral tests will only pick up cases of Covid-19, not colds or flu. 

Speaking to TheJournal.ie, lecturer in immunology and host-microbe interactions at the Department of Biological Sciences at the University of Limerick, Dr Elizabeth J Ryan, said that viral tests for current cases of Covid-19 are “very specific” for material that is “only expressed by the Covid-19 virus”.

“Having a cold or flu won’t change that,” Dr Ryan said.

Similarly, Professor Kingston Mills, a professor of experimental immunology in Trinity’s School of Biochemistry and Immunology, told TheJournal.ie that the viral test for current Covid-19 cases is “very, very specific”.

The test is based on material that is unique to the virus, Professor Mills said, so it’s “not going to pick up influenza or common cold virus”.

The page on the CDC website that gives details on tests for past infection was last updated on 30 June and says a “positive test result shows you may have antibodies from an infection with the virus that causes Covid-19”.

“However, there is a chance a positive result means that you have antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold,” the CDC statement says.

The CDC is warning here that an antibody test – a test for past infection with Covid-19 – could have the potential to return a false positive result for Covid-19 if the person tested has created antibodies for another infection caused by a different type of coronavirus, like one that can cause a cold.

Dr Ryan said that some antibody tests which are not properly validated are not specific enough for the virus they are testing for and can pick up other antibodies instead, giving a “false positive test”. 

Validated tests are ones which have been rigorously tested in a laboratory environment, such as by verifying the test through samples of the virus that have already tested positive.

The World Health Organization has warned against using rapid tests that have not been validated, except in a research setting.

“Coronavirus, if you are infected with the virus, will be in your system for a couple of weeks. Your immune system will clear this, and in the process of clearing it, you will mount an antibody response – an immune response,” Dr Ryan said.

“Most people will produce antibodies when they’re infected. The antibody test is looking for those antibodies.

The antibody tests are not all equal. Some tests from different manufacturers have been properly validated, and those properly validated tests will test the specific antibody to Covid-19. Other coronaviruses [than Covid-19] exist and can cause the common cold. We’ve all had them, probably, at some point, so we will all have antibodies in our system to those viruses.

“Tests that have been properly validated and gone through proper regulatory channels will account for that and we will look for that when we’re developing those tests, to make sure that there is no cross-reactivity between the virus that causes Covid-19 and other coronaviruses.

You will only have antibodies to Covid-19 if you have had the virus,” she said. 

Some private laboratories in Ireland are offering antibody testing for Covid-19 at a charge, but antibody testing for Covid-19 is not being carried out widely among the public in Ireland at present.

The HSE and the UCD National Virus Reference Laboratory are currently using antibody testing for a study looking to estimate the number of people in the country who have been infected with Covid-19.

Antibody tests are not currently counted towards daily reporting of the number of Covid-19 cases in Ireland. The number of cases is counted using the results of polymerase chain reaction (PCR) tests – that is, tests that identify an existing infection with Covid-19.

Professor Mills also said that 10-minute or strip tests for antibody testing which have not been properly validated could falsely return a positive result due to colds, but that properly validated tests for antibodies are specific to Covid-19.

The HSE antibody study uses properly validated tests. Participants have a blood test carried out in a local centre with a nurse, and the blood sample is sent to the National Virus Reference Laboratory in UCD, where it is tested for antibodies.

Although people in Ireland can obtain a flu vaccine, there is no vaccine against the common cold.

Colds can be causes by several different types of viruses, including rhinoviruses, adenoviruses, and coronaviruses.

The Verdict

Antibody testing and viral testing are carried out differently and look for different things.

Antibody testing identifies past infection with a virus by looking for antibodies in blood that the body has produced against the virus. Viral testing looks for an existing case of the virus, and is done with a sample from the respiratory system – that is, a swab to the throat and/or nose.

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The statement from the CDC said that an antibody test – a test for a past infection – for Covid-19 could pick up antibodies for other types of coronaviruses, such as a cold.

The CDC’s statement has been used to spread a claim that tests for Covid-19 – including tests for existing cases – will return a positive result if the person has a cold or flu, or if they have ever had a vaccine against the cold or flu.

Although there is a chance that an antibody test that is not properly validated can return a false positive due to a cold that was caused by a coronavirus, a test that is properly validated will only identify antibodies that the body has produced against Covid-19.

The flu is caused by a different virus entirely, and cannot cause a false positive result.

An antibody test will not return a positive due to a person previously being vaccinated against the flu. Additionally, a vaccine against the common cold does not exist.

Testing for an existing case of Covid-19 will not return a positive result due to a cold or flu

As a result, we rate the claim that a common cold can cause a positive result for a Covid-19 test:  FALSE.

As per our verdict guide, this means: The claim is inaccurate.

TheJournal.ie’s FactCheck is a signatory to the International Fact-Checking Network’s Code of Principles. You can read it here. For information on how FactCheck works, what the verdicts mean, and how you can take part, check out our Reader’s Guide here. You can read about the team of editors and reporters who work on the factchecks here.

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