Ongoing COVID-19 infections are detected with a swab from the nose, throat or lungs (Kirill KUDRYAVTSEV/ AFP)

Ongoing COVID-19 infections are detected with a viral test, not a blood check for antibodies

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Widely shared social media posts claim the US Centers for Disease Control and Prevention (CDC) says that positive COVID-19 results simply mean people have previously contracted a cold. In reality, the claim is a misinterpretation of a paragraph discussing antibody tests on the CDC's website. Health experts say ongoing COVID-19 infections are detected with viral tests, not an analysis of antibodies.

“SHOCKER: CDC admits COVID-19 ‘positive result’ just means you've previously contracted the common cold,” one Facebook post says.

A screenshot of a false Facebook post, taken on July 23, 2020

Similar Facebook posts have been shared South Africa here and in the US archived here and here.

This Polish Facebook post shared a screenshot of the actual paragraph from the CDC's website. The caption translates to: "American CDC reports that a positive test result (allegedly for COVID-19) could mean that you had any other cold or flu caused by other coronaviruses".

Screenshot of the misleading Polish Facebook post, taken on July 27, 2020

The screenshot used in the post can be found on the CDC website under the topic: “Test for Past Infection (Antibody Test)”.

A screenshot of the paragraph (in the red rectangle) on CDC website, taken on July 24, 2020

But health experts say the social media posts have misinterpreted the paragraph -- including Kenyan doctor Githinji Gitahi, a board member of the Africa Centres for Disease Control and Prevention. The institution is part of the African Union and unrelated to the American CDC.

Viral test to confirm COVID-19

Gitahi, who also runs the medical NGO Amref Health Africa, told AFP Fact Check there are two types of tests for COVID-19 -- the direct or viral test, and the indirect test, also referred to as the antibody test.

Blood samples are used to check for traces of antibodies, which indicate if someone was previously infected with SARS-CoV-2 -- the virus that causes COVID-19.

The viral test, meanwhile, uses samples from the respiratory system, such as a swab from the inside of your nose, throat or lungs to detect an ongoing COVID-19 infection.

"Since there is currently no antibody test that is specific to antibodies produced to fight SARS-CoV-2, there is a likelihood to test positive if one tests for antibodies but it might not be because of COVID-19 but because the antibodies came from other coronaviruses, even the one that causes the common cold," Gitahi said.

“Because of this confusion, we do not use the antibody test to confirm COVID-19 infection but we use the viral test, which is conducted in the nose or throat with swabs or in the lungs.”  

Antibody tests used for surveillance

The US-based CDC told AFP Fact Check that a viral test will not produce a positive result for COVID-19 from the traces of other human coronaviruses.

"The viral test detects the SARS-CoV-2 virus in respiratory samples. The presence of other human coronaviruses in a respiratory sample do not produce a positive result on a diagnostic test for SARS-CoV-2," spokeswoman Jasmine Reed said in an email.  

 "A positive result on an antibody test might mean that a person had a past infection with SARS-CoV-2. But there is a small chance that antibodies from an infection with a different coronavirus, such as the one that causes the common cold, might cause a positive result on a SARS-CoV-2 antibody test."

WHO acknowledges that antibody tests such as serology testing can be used in COVID-19  surveillance to measure the extent of infection in the population.

But it cautions against the uncertainty of the test in terms of sensitivity and specificity, and its ability to “assess cross-reactivity” with other coronaviruses.