What our experts say
The blood of people who have received a COVID-19 vaccine is not visibly darker than the blood of vaccinated people.
Human blood is red because of a protein called hemoglobin that is inside it. This happens because hemoglobin contains a compound called heme that is colored red and helps transport oxygen throughout the body. Without hemoglobin and heme in it, blood would likely appear to be a lighter, yellower, clearer tone—the color of plasma—which is the part of blood that that is left when hemoglobin/red blood cells are removed. People who are vaccinated against COVID-19 do not have lower levels of hemoglobin in their blood and COVID-19 vaccines do not change the color of blood.
Blood can appear to be black when it dries out and hemoglobin breaks down through a chemical change. For example, this process can help forensic scientists identify when blood might have been left at a crime scene due to its dryness and color. Blood can also appear to be very dark or black (but is actually more likely to be old, brown blood) during women's menstruation cycles, during the first month to six weeks after childbirth or miscarriages, or other reasons.
Context and background
Several Instagram and Facebook posts (which have been flagged as false information) are incorrectly claiming that blood from vaccinated people is black or a much darker red.
The much more likely scenario is that the posts show two types of blood. The first vial of lighter red blood is arterial blood that has been oxygenated and is found in the pulmonary vein, left heart chambers, and the arteries. This blood is pumped from the heart throughout the body.
Conversely, venous blood carries blood to the heart and is a darker red color. This blood has less oxygen and more carbon dioxide because of the gas exchange that happens in the body through circulation.
Despite the presence of these rumors, the blood of vaccinated people appears no different than that of unvaccinated people and vaccines do not deplete hemoglobin levels.
Blood that is donated and used is also not marked as vaccinated or unvaccinated so there is no way to tell if the donor has been immunized
Used with Permission from Health Desk, a public health hub that explains emerging COVID-19 science.