COVID-19 can lower your red blood cell count, and the effect is more common than many people realize. In one hospital study of over 200 patients, roughly six in ten had anemia at the time they were admitted, compared with fewer than half of patients who had similar symptoms but tested negative for the virus. The drop happens through several overlapping pathways, from inflammation that suppresses new red blood cell production to direct damage the virus inflicts on the cells themselves, and the severity of the change tracks closely with how sick a patient becomes.
How Common Is Anemia During a COVID Infection
Anemia, the clinical term for having too few red blood cells or too little hemoglobin, showed up frequently in hospitalized COVID patients throughout the pandemic. An Italian observational study found that 61% of 206 hospitalized COVID patients were anemic on admission, a rate significantly higher than the 45% seen in a control group of patients with similar respiratory symptoms who tested negative for SARS-CoV-2.1PubMed Central. Anemia in patients with Covid-19: pathogenesis and clinical significance Most of these cases were mild and driven by inflammation, sometimes compounded by iron or vitamin deficiencies. Women in that study had lower hemoglobin levels than men and a higher rate of moderate-to-severe anemia.
A large meta-analysis pooling data from many studies found that the average hemoglobin level across COVID patients of all ages was about 130 g/L, which sits at the low end of the normal range. Patients with severe disease had measurably lower hemoglobin and lower red blood cell counts than those with moderate illness. Older patients, those with diabetes or hypertension, and those admitted to intensive care fared worse on these blood markers.2PubMed Central. Anemia and iron metabolism in COVID-19: a systematic review and meta-analysis
Anemia as a Risk Factor for Worse Outcomes
Having low red blood cell counts at admission was not just a side effect of feeling sick. It was an independent predictor of how badly COVID would go. A retrospective study from Wuhan found that anemic COVID patients were about three-and-a-half to nearly four times more likely to develop severe illness than non-anemic patients, even after adjusting for other baseline health factors.3PubMed Central. Anemia is associated with severe illness in COVID-19: A retrospective cohort study A separate cross-sectional analysis of 784 patients found a significant link between anemia and death from COVID in both men and women. That association held even in the subgroup of patients who had no other comorbidities besides anemia, suggesting the relationship was not simply explained by sicker patients being more likely to be anemic for other reasons.4PubMed Central. Relationship of anemia with COVID-19 deaths: A retrospective cross-sectional study
This does not mean anemia caused the deaths. But it does mean that if you were hospitalized with COVID and your hemoglobin was already low, your chances of a rough course were considerably higher. Clinicians learned early in the pandemic to pay close attention to hemoglobin levels on admission as one of several warning signs.
Why COVID Drives Red Blood Cells Down
The virus attacks red blood cell numbers and function through at least three distinct routes, and they tend to operate at the same time.
The first and most common is inflammation. When SARS-CoV-2 triggers a strong immune response, your body ramps up production of inflammatory signaling molecules. One downstream effect is a surge in hepcidin, a hormone that locks iron inside cells and pulls it out of circulation. With less iron available for making new red blood cells, production in the bone marrow slows down. Ferritin, the protein that stores iron, shoots up because the body is hoarding iron rather than using it. The meta-analysis mentioned earlier found that average ferritin in COVID patients was dramatically elevated, and it climbed even higher in severe cases.2PubMed Central. Anemia and iron metabolism in COVID-19: a systematic review and meta-analysis A review of iron-related changes in COVID confirmed a consistent pattern: low circulating iron, low transferrin (the protein that carries iron in the blood), and sky-high ferritin and hepcidin.5PubMed Central. Iron and iron-related proteins in COVID-19 This is essentially the body stealing its own iron supply to starve the virus, with the side effect of choking off red blood cell production.
The second route is direct damage to existing red blood cells. Proteomics research on blood from COVID patients found that structural membrane proteins in red blood cells showed signs of oxidative damage, with increased oxidation of key cytoskeletal proteins that give cells their shape and flexibility.6PubMed Central. Evidence for Structural Protein Damage and Membrane Lipid Remodeling in Red Blood Cells from COVID-19 Patients This damage makes red blood cells more fragile and shorter-lived. A broader review of the literature concluded that SARS-CoV-2 infection causes measurable harm to red blood cells, changing their shape, their ability to deform as they squeeze through tiny capillaries, and their tendency to clump together.7PubMed Central. Morphology and Function of Red Blood Cells in COVID-19 Patients: Current Overview
The third route involves the immune system turning against its own red blood cells. COVID has been linked to autoimmune hemolytic anemia, a condition where the body produces antibodies that mistakenly target and destroy its own red blood cells. This can happen either through the formation of autoantibodies or through direct injury mediated by proteins on the red blood cell surface.8PubMed Central. Hemolytic anemia in COVID-19 Case series have documented this complication across multiple hospitals, with the autoantibody-driven pathway appearing to be the most frequently reported mechanism.9PubMed Central. Hemolytic Anemia Complicating COVID-19 Infection
What Happens to Red Blood Cells Beyond Just the Count
Counting red blood cells tells you part of the story, but the cells that remain in circulation are often not working normally either. Research has documented changes in the physical properties of red blood cells that go well beyond just having fewer of them.
Healthy red blood cells are remarkably flexible. They can squeeze through capillaries narrower than their own diameter, which is essential for delivering oxygen to every tissue. In COVID patients, the cells become stiffer and less deformable. They also tend to clump together more aggressively. Simulation studies showed that the elevated fibrinogen levels seen in COVID can promote the formation of red blood cell clusters at low flow rates, which raises blood viscosity.10PubMed Central. Circulating cell clusters aggravate the hemorheological abnormalities in COVID-19 Thicker, stickier blood flowing through already-inflamed blood vessels is a recipe for clotting problems, and this partly explains the unusually high rate of blood clots seen in severe COVID.
The interaction between hypoxia (low oxygen), inflammation, and blood cell production creates a feedback loop during severe infection. The body is simultaneously short on oxygen, short on functional red blood cells, and dealing with an immune system that is disrupting normal blood production.11PubMed Central. Haematological Manifestations of SARS-CoV-2: Insights into Erythropoiesis, Hepcidin Regulation, and Cytokine Storm Research tracking erythropoietin, the hormone that signals the body to make more red blood cells, found that levels were actually lower in ICU patients on admission than in less severe cases. After about a week, erythropoietin rose in the sickest patients as their bodies finally responded to the oxygen deficit.12PubMed Central. The role of hypoxia and inflammation in the regulation of iron metabolism and erythropoiesis in COVID-19: The IRONCOVID study The delayed response hints that the inflammatory storm in early severe COVID actively suppresses the bone marrow’s natural attempt to compensate.
The Oxygen Delivery Problem
Having fewer red blood cells is bad enough, but COVID also appears to change how effectively hemoglobin picks up and releases oxygen. Several studies investigated the oxygen dissociation curve in COVID patients, measuring how tightly hemoglobin binds oxygen at different pressures. In the largest of these, researchers found a clear “left shift” when measurements were taken from fresh blood, meaning hemoglobin was gripping oxygen more tightly than normal.13PubMed Central. The oxygen dissociation curve of blood in COVID-19
This is a double-edged finding. A tighter grip on oxygen could help the lungs load up hemoglobin even when the lungs are damaged and oxygen is scarce. But it also means hemoglobin is less willing to release that oxygen when it reaches tissues that desperately need it. A study of patients with severe COVID respiratory failure found that their hemoglobin-oxygen affinity was significantly higher than in a control group of patients with other causes of severe respiratory failure.14PubMed Central. A left shift in the oxyhaemoglobin dissociation curve in patients with severe coronavirus disease 2019 (COVID-19) The researchers speculated that the prolonged disease course allowed a kind of acclimatization to low oxygen levels. An updated review noted that while some studies found no changes, three large studies found increased affinity that was associated with better outcomes, suggesting this shift may sometimes be a helpful adaptation rather than a purely harmful one.15PubMed Central. The oxygen dissociation curve of blood in COVID-19-An update
Red Blood Cell Distribution Width as an Early Warning Sign
One of the more practical discoveries to come out of pandemic-era blood research involves a test that is already part of a standard blood panel. Red blood cell distribution width, or RDW, measures how much variation there is in the size of your red blood cells. A higher number means more size variation, which usually signals that something is disrupting normal red blood cell production or lifespan.
In COVID patients, RDW proved to be a surprisingly strong predictor of who would get critically ill. One study found that patients with an RDW above 11.5% were far more likely to develop critical illness than those below that threshold, and elevated RDW was an independent predictor of critical COVID even after accounting for other factors.16PubMed Central. Red blood cell distribution width: A severity indicator in patients with COVID-19 A prospective study from Cincinnati saw a progressive climb in RDW as disease severity increased, with elevated values associated with a nine-fold increase in the odds of severe illness.17PubMed Central. Red Blood Cell Distribution Width (RDW) Predicts COVID-19 Severity: A Prospective, Observational Study from the Cincinnati SARS-CoV-2 Emergency Department Cohort A large study published in JAMA Network Open added a mortality dimension: patients admitted with an RDW above 14.5% had a 31% mortality rate, roughly three times the 11% rate seen in patients with lower values. If RDW increased during hospitalization, mortality risk climbed further.18JAMA Network Open. Association of Red Blood Cell Distribution Width With Mortality Risk in Hospitalized Adults With SARS-CoV-2 Infection
The reason RDW works so well as a marker is probably that it captures the combined impact of all those mechanisms at once: inflammation, iron disruption, oxidative damage, and impaired production all produce red blood cells of uneven sizes. It is cheap, available on any routine blood draw, and does not require any special equipment. That combination made it genuinely useful in overstretched hospitals trying to triage patients quickly.
How COVID Compares to Influenza
An early head-to-head comparison of COVID and influenza patients found an interesting pattern. At the time they first showed up to the hospital, COVID patients actually had higher red blood cell counts and hemoglobin levels than flu patients. But over the following days, COVID patients’ hemoglobin and red blood cell counts dropped more steeply, eventually falling to the same depressed levels seen in influenza patients during the later stages of illness.19The Lancet. Temporal changes in laboratory markers of COVID-19 and comparison with influenza This faster decline likely reflects the additional mechanisms COVID brings to bear on red blood cells, especially the direct cellular damage and the fierce inflammatory iron-hoarding response. Both viruses cause anemia of inflammation, but COVID appears to do it more aggressively.
Do Red Blood Cell Changes Persist After Recovery
For most people with mild or moderate COVID, red blood cell counts return to normal as the infection resolves and inflammation subsides. But the picture is more complicated in people who develop lingering symptoms. Research into long COVID has found that some hematological changes can persist well beyond the acute phase, including alterations in red blood cells, ongoing anemia, and elevated inflammatory markers like ferritin.20PubMed Central. Hematological alterations associated with long COVID-19
A study of children and adolescents who had recovered from COVID offered a window into the timeline of recovery. Researchers found that young people who had seroconverted (developed antibodies to the virus) within the past six months showed significantly increased red blood cell deformation compared to those who had never been infected. But in those whose infection was more than six months in the past, red blood cell deformation had returned to normal and was no longer different from uninfected participants.21Scientific Reports. Increased red blood cell deformation in children and adolescents after SARS-CoV-2 infection This suggests that for younger, otherwise healthy people, the physical damage COVID inflicts on red blood cells is largely reversible within about six months. Whether the same timeline applies to older adults or those who had severe infections is less clear.
People with Pre-Existing Blood Conditions
If you already have a condition that affects your red blood cells, COVID can hit harder. Sickle cell disease, which causes red blood cells to be abnormally shaped and prone to clumping, is the most studied example. A critical appraisal of the literature found that adults with sickle cell disease typically had a mild-to-moderate COVID course, but their risk of being hospitalized was two to seven times higher than the general population, with a modestly increased risk of death.22PubMed Central. Clinical outcomes of COVID-19 in patients with sickle cell disease and sickle cell trait: A critical appraisal of the literature
A more recent systematic review and meta-analysis confirmed the hospitalization risk and added important detail. People with full sickle cell disease had about an eight-fold higher risk of hospitalization compared to controls, though once hospitalized, their risk of dying was not significantly different. People who carried just the sickle cell trait (one copy of the gene rather than two) did not have a significantly higher hospitalization risk, but they did have about a 43% higher risk of death from COVID.23PubMed. COVID-19 outcomes among patients with sickle cell disease or sickle cell trait compared to the general population: a systematic review and meta-analyses That last finding is striking because sickle cell trait is usually considered benign in everyday life, and many carriers do not know they have it.
COVID Vaccines and Red Blood Cells
Early in the vaccine rollout, scattered case reports described patients developing autoimmune hemolytic anemia after receiving COVID vaccines. In one well-documented case, a patient developed the condition about two months after a first mRNA vaccine dose. The researchers proposed that the immune response triggered by the vaccine could, in rare instances, activate immune cells that cross-react with proteins on the red blood cell surface.24PubMed Central. Autoimmune Hemolytic Anemia Secondary to COVID-19 Vaccine: A Case Report Other case reports documented similar events after second doses of mRNA vaccines.25PubMed Central. A Case of Autoimmune Hemolytic Anemia Following COVID-19 Messenger Ribonucleic Acid Vaccination
Importantly, a large population-based Korean cohort study put these case reports in context. It found that vaccinated people actually had a lower risk of hemolytic anemia in the three months after vaccination compared to unvaccinated individuals, regardless of whether they received mRNA or viral vector vaccines. Meanwhile, COVID infection itself was associated with a roughly seven-fold increase in hemolytic anemia risk.26medRxiv. Hematologic abnormalities after COVID-19 vaccination: A large Korean population-based cohort study So while the rare case reports are real, the population-level data strongly suggests that the disease is far more dangerous to your red blood cells than the vaccine is. The case reports describe events that are manageable with treatment and occur at a rate that is dwarfed by the red blood cell destruction caused by the infection itself.