Blood bags are pictured at a hemotherapy center in Santa Cruz de Tenerife on the Canary Island of Tenerife in 2018 ( AFP / Desiree Martin)

Covid-19 vaccines do not affect blood color

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Social media posts claim that Covid-19 vaccines negatively affect blood, using a photo of two bags of the liquid -- one lighter red than the other -- as evidence. This is false; experts say the difference in color is due to oxygenation levels, and that the shots will not impact blood color.

"Left vs. Right= non vax vs. Vax," says text accompanying the image of bags of blood in a September 17, 2021 Instagram post, which also included the comment: "Looks like a bag of blood clots."

Screenshot of an Instagram post taken on September 21, 2021

The image was also shared on Twitter here and here.

AFP Fact Check has previously debunked claims that vaccine recipients cannot donate plasma, and that the shots permanently alter blood cells. The assertion that the vaccines impact blood color is also inaccurate.

"This has absolutely nothing to do with vaccination," Dr PJ Utz, a professor of immunology and rheumatology who runs a Stanford University research lab, told AFP.

"The color is dictated by how much oxygen is in the blood, and that is a highly variable thing," he said. "If you were to line up 15 packs of blood you would see varying shades of red so there's absolutely nothing to worry about with those blood samples."

Covid-19 vaccines based on the viral vector technology such as AstraZeneca's have been linked to cases of blood clots, but these events remain extremely rare.

Utz said: "If there's a message out there, it's that if you have to choose between vaccination and infection, you take vaccination 100 percent of the time," pointing to a study showing that being infected with Covid-19 increased likelihood of developing autoantibodies that can cause blood clots.

Dan Milner, chief medical officer at the American Society for Clinical Pathology, also said that the variation in blood color comes from oxygenation levels, which change based on "different levels of tissue usage, if they (the patient) have more muscle, more fat, whether the blood is from a dominant or a non-dominant arm, etc."

He explained that blood is always drawn from veins, which carry deoxygenated blood back towards the heart, and put in blood bags that are hermetically sealed. This is why blood in donation bags is quite dark, particularly when compared to the bright red oxygenated blood visible after a cut.

Some commenters suggested that the difference in blood color in the photo might be due to one bag containing blood from a vein, and the other containing blood from an artery, which carries oxygenated blood away from the heart. This is also false, according to Milner, who said that any blood drawn from arteries would be "very unhealthy for the patient."

He also said the that the bag pictured in the Instagram post would not have contained clotted blood, as the Instagram post suggested.

"You can't draw blood that's clotting because it would stick in the needle and it wouldn't fill the bag. So the idea that there's a clot in there is completely preposterous," Milner said.

Gregory Poland, a Mayo Clinic expert in viruses and vaccines, looked at the image and called the post "misinformation," adding that vaccines do not cause what is pictured.

AFP Fact Check has debunked other inaccurate claims related to Covid-19 and vaccines here.

September 27, 2021 This article was updated to add studies on Covid-19 vaccines in paragraph seven.