COVID-19 kills primarily by destroying the lungs’ ability to deliver oxygen to the blood, but it can also trigger fatal damage to the heart, brain, kidneys, and other organs. The virus uses a combination of direct cell destruction, an overblown immune response, and widespread blood clotting to overwhelm the body. In the prevaccination era, the fatality rate ranged from 0.01% in children to nearly 28% in adults over 85.
How the Virus Gets Inside Cells
SARS-CoV-2 enters human cells by latching onto a protein called ACE2, which sits on the surface of cells throughout the body. ACE2 is found not just in the lungs but also in the heart, kidneys, liver, digestive tract, and brain. This wide distribution is why COVID-19 can cause damage far beyond the respiratory system. Once the virus binds to ACE2, a second protein on the cell surface helps it fuse with the cell membrane and slip inside, where it hijacks the cell’s machinery to make copies of itself.
Lung Damage and Oxygen Failure
The most common cause of death is respiratory failure. Deep inside the lungs, tiny air sacs called alveoli are separated from blood vessels by an extremely thin barrier. Oxygen crosses this barrier with every breath. COVID-19 attacks both sides of it: the virus directly destroys the cells lining the air sacs, and the body’s inflammatory response damages the blood vessel walls on the other side.
This two-pronged assault causes what’s known as diffuse alveolar damage. The air sacs flood with fluid, fibrin (a clotting protein), and debris. Autopsy studies of COVID-19 patients have found dense fluid buildup, tiny blood clots lodged in the lung’s smallest vessels, and early scarring from collagen deposits. The result is that oxygen can no longer pass efficiently into the bloodstream, and carbon dioxide can’t get out. Even with mechanical ventilation, this level of lung destruction can be unsurvivable. Among patients admitted to an ICU in the prevaccination period, the fatality rate was 44.2%.
The Immune System Turns on the Body
In many fatal cases, the immune response itself becomes the killer. The body detects the virus and releases signaling molecules called cytokines to coordinate the fight. In severe COVID-19, this system spirals out of control into what’s called a cytokine storm. Blood levels of one inflammatory signal in particular, IL-6, are strongly correlated with whether a patient survives or dies.
IL-6 triggers a chain reaction that hyperactivates inflammation not just in immune cells but in the cells lining the air sacs and blood vessels themselves. This creates a feedback loop: damaged tissue triggers more inflammation, which damages more tissue. The inflammation isn’t confined to the lungs. It spills into the bloodstream and can affect every organ in the body, driving the kind of multi-organ failure that makes severe COVID-19 so difficult to treat.
Blood Clots From Head to Toe
One of COVID-19’s most dangerous features is its ability to cause abnormal clotting throughout the vascular system. The virus damages the inner lining of blood vessels directly, and the resulting inflammation makes blood more prone to clotting. These clots form at every scale, from microscopic clots in the lung’s tiniest capillaries to large clots that cause strokes and heart attacks.
Patients with severe COVID-19 show markedly elevated levels of D-dimer, a protein fragment that indicates clots are forming and breaking down throughout the body. These circulating clots can lodge in the brain, causing neurological damage. They can block blood flow to the kidneys, causing acute kidney injury. Even the spike protein itself, separated from the virus, can cross into the brain’s blood vessels and trigger inflammatory and clotting responses in the vessel walls. This coagulopathy, as researchers call it, is a major reason COVID-19 can kill people whose lungs seem to be holding up relatively well.
Heart Damage
COVID-19 can inflame the heart muscle directly, a condition called myocarditis. This occurred in roughly 2 to 4 out of every 1,000 hospitalized COVID-19 patients and can lead to heart failure or sudden cardiac death. Myocardial injury with swelling has been identified even in patients with mild or asymptomatic infections, meaning heart damage doesn’t require a severe case. The combination of direct viral invasion, inflammatory damage, and clot-related blockages to the heart’s blood supply makes cardiac complications a significant contributor to COVID-19 deaths.
Who Faces the Highest Risk
Age is the single strongest predictor of dying from COVID-19. Data from the prevaccination period in the United States shows how steeply risk climbs with age:
- Children 1 to 14: 0.01% fatality rate
- Adults 25 to 34: 0.08%
- Adults 45 to 54: 0.6%
- Adults 65 to 74: 5.7%
- Adults 75 to 84: 14.4%
- Adults over 85: 27.9%
Underlying health conditions compound this risk significantly. Among COVID-19 deaths in New York City, 55.4% of patients had high blood pressure, 37.3% had diabetes, and 18.5% had high cholesterol. After adjusting for age and smoking, cancer tripled the risk of a fatal outcome, chronic obstructive pulmonary disease (COPD) roughly doubled it, and both diabetes and high blood pressure increased it by about 60%. Chronic kidney disease, obesity, and cardiovascular disease also appeared consistently among the most common conditions in patients who died.
The reason these conditions are so dangerous in combination with COVID-19 comes back to the same mechanisms. Diabetes and high blood pressure already stress blood vessels and impair immune regulation. Obesity increases baseline inflammation. Chronic lung disease means less respiratory reserve to withstand viral damage. Each of these conditions narrows the margin the body has to absorb the hit.
How Vaccination Changed the Numbers
Vaccination dramatically reduces the risk of dying from COVID-19. CDC data from late 2022 found that unvaccinated people died at 14 times the rate of those who had received an updated booster dose. For adults aged 65 to 79, unvaccinated individuals died at nearly 24 times the rate of boosted individuals. Even older adults over 80, the highest-risk group, saw their death rate drop by a factor of 10 with an updated booster compared to no vaccination at all.
During the late Omicron BA.4/BA.5 wave, the average weekly death rate among boosted individuals was 0.1 per 100,000 people, compared to substantially higher rates among the unvaccinated. The crude vaccine effectiveness against death was estimated at 93% for recently boosted individuals. Vaccines work primarily by training the immune system to mount a faster, more targeted response, reducing the chance that the virus replicates enough to trigger the cascade of lung destruction, runaway inflammation, and clotting that leads to death.