Flu shots aren’t causing false positive COVID-19 tests

A recent Facebook post is fanning the flames of a conspiracy theory that officials are overstating the number of COVID-19 cases. 

"I had a phone appointment with my Dr this afternoon, and I asked the question about testing," it says. "He said, ‘If you have had a flu shot in the last 3-5 years, you will probably test positive. Coronavirus antibodies have been in the vaccines since H1N1.’ This would account for all of ‘tested positive (with no symptoms) results."

This post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.) 

We reached out to medical experts about the post. 

Davidson Hamer, a global health and medicine professor at Boston University as well as a faculty member of the National Emerging Infectious Diseases Laboratory, called it "nonsense."  

"Coronaviruses have a completely different antigenic (surface protein and glycoprotein structure) from influenza viruses," he told PolitiFact in an email. "They both cause similar syndromes but they are distinct classes of viruses and there is no evidence that I am aware of that would lead to cross reactions between immune responses to the two vaccines." 

Plus, he added, there’s "definitely no trace of coronaviruses in influenza vaccines."

Richard Watanabe, a preventive medicine professor at the University of Southern California, told us that since there’s no evidence the novel coronavirus existed in humans before last year, "there is no way antibodies existed before then and therefore could not have been identified and isolated." 

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"But more importantly," he said, "this post seems to imply that vaccines work by directly giving you antibodies against whatever bug. That’s not how vaccines work. Vaccines give you an inactive form of the bug to allow your immune system to create antibodies." (You can read more about how vaccines do work here.)

Several other fact-checkers have also investigated whether a flu shot can result in false positives for COVID-19, and all came to the same conclusion: no. 

Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital in Philadelphia, told FactCheck.org that there’s no cross reaction between influenza and the new coronavirus, meaning flu traces wouldn’t cause someone to test positive for COVID-19.

A spokesperson for the Food and Drug Administration also told FactCheck.org that all FDA-approved COVID-19 tests are checked for cross-reactivity with influenza virus and influenza antibodies and so far, no cross-reactivity has occurred.

We rate this Facebook post Pants on Fire.