COVID arrived as a brand-new virus in a world where nobody had immunity, so it reached enormous numbers of people at once. Even a small share of infections turning fatal added up to millions of deaths. In many of the worst cases, the deadliest damage came less from the virus itself than from the body’s own reaction to it, with inflammation and blood clotting spinning out of control. And who died depended heavily on age, health, and circumstance, which is why the toll fell so unevenly.
How the Virus Gets Into Cells Across the Body
The COVID virus gets into human cells by latching onto a protein on their surface called ACE2.1Cell. SARS-CoV-2 Cell Entry Depends on ACE2 and TMPRSS2 and Is Blocked by a Clinically Proven Protease Inhibitor Cells without ACE2 resisted infection, while the same cells given ACE2 became easy to infect.2Cell. SARS-CoV-2 Cell Entry Depends on ACE2 and TMPRSS2 and Is Blocked by a Clinically Proven Protease Inhibitor The virus also needs a second helper protein on the cell to fuse and get inside.3Cell. SARS-CoV-2 Cell Entry Depends on ACE2 and TMPRSS2 and Is Blocked by a Clinically Proven Protease Inhibitor
Many cold viruses stay mostly in the nose and throat. COVID could go further because the doorway is found all over the body. Heart muscle carries a high density of it, which may be why heart damage often appeared early in severe illness.4PubMed Central. Outcomes of Hospitalized Patients With COVID-19 With Acute Kidney Injury and Acute Cardiac Injury In people who died of COVID, virus was found inside the cells lining blood vessels in the lungs, kidneys, and intestine.5PubMed Central. The Emerging Threat of (Micro)Thrombosis in COVID-19 and Its Therapeutic Implications A virus that infects blood vessel linings travels with the blood to every organ. That helps explain the kidney, heart, and clotting problems that a cold never causes.
Timing mattered too. Flu meets bodies that partly recognize it from earlier years, but no adult had met this virus before 2020.
When the Immune System and Blood Clotting Turn Dangerous
In severe COVID, the immune system can release a flood of inflammatory signals, sometimes called a cytokine storm.6PubMed Central. Mechanism of COVID-19 Causing ARDS: Exploring the Possibility of Preventing and Treating SARS-CoV-2 This runaway inflammation breaks down the thin barriers in the lungs’ air sacs and the tiny blood vessels around them.7PubMed Central. Mechanism of COVID-19 Causing ARDS: Exploring the Possibility of Preventing and Treating SARS-CoV-2 Fluid then leaks into the air sacs, and the blood can no longer pick up enough oxygen.8PubMed Central. Mechanism of COVID-19 Causing ARDS: Exploring the Possibility of Preventing and Treating SARS-CoV-2 Patients develop a form of lung failure that was a leading way people died.
Inflammation normally helps clear an infection and then switches off. In severe COVID it stays on and harms healthy tissue. That explains why many patients worsened in the second week, after the virus itself had peaked.
Damaged blood vessels set off clotting throughout the body. Injured vessel linings trigger countless tiny clots in small vessels, because activated platelets, extra clotting proteins, and immune cells together push the blood toward clotting.9PubMed Central. Damage to endothelial barriers and its contribution to long COVID Severe COVID also raised the risk of clots in veins and arteries, not just tiny vessels.10PubMed Central. Endothelial dysfunction and immunothrombosis as key pathogenic mechanisms in COVID-19 Blocked blood flow and broken vessel walls could injure the lungs, brain, kidneys, and gut at the same time.11PubMed Central. Damage to endothelial barriers and its contribution to long COVID Many deaths came from several organs failing together, not from the lungs alone.
For patients, this looked like strokes, heart attacks, or clots in the legs and lungs. Many hospitals began giving blood thinners to admitted patients.
Why Some Patients Didn’t Feel How Sick They Were
Some COVID patients had dangerously low oxygen without feeling short of breath. The urge to gasp for air is driven mostly by carbon dioxide building up, and many of these patients’ carbon dioxide stayed low.12PubMed Central. Why COVID-19 Silent Hypoxemia Is Baffling to Physicians Age and diabetes dull the alarm further. The body’s breathing response to low oxygen is about half as strong in people over 65.13PubMed Central. Why COVID-19 Silent Hypoxemia Is Baffling to Physicians In people with diabetes that response is cut by more than half, and they are less able to sense breathing problems.14PubMed Central. Why COVID-19 Silent Hypoxemia Is Baffling to Physicians The exact causes are still debated, though damage to lung blood vessels is the best-supported explanation.15PubMed. Pathophysiology of silent hypoxia in COVID-19: Truth and mystery Patients stayed home feeling okay while their lungs worsened, and by the time breathlessness came, damage could be advanced.
Checking oxygen at home with a fingertip meter can help, with limits. Healthy people usually read about 95% to 100%, and readings that stay below about 92% usually warrant calling a doctor. Darker skin can make meters read falsely high, so watch the trend as well as any single number, and don’t let a normal reading override warning signs. The meters also become less accurate when readings fall below 80%.16PubMed Central. Why COVID-19 Silent Hypoxemia Is Baffling to Physicians A reading that keeps dropping is a reason to seek care even if you feel all right.
Why Age Was the Biggest Risk Factor
With age, the immune system gets slower at mounting a targeted defense against new germs. Older people with COVID often had weaker T-cell responses and poorer antibody production against the virus.17PubMed Central. Remodeling of the Immune Response With Aging: Immunosenescence and Its Potential Impact on COVID-19 Immune Response At the same time, older bodies tend to carry low-level background inflammation.18PubMed Central. Immunosenescence and COVID-19 When the virus arrived, that background inflammation fed an exaggerated immune reaction and worse outcomes.19PubMed Central. SARS-CoV-2, COVID-19 and the Ageing Immune System How much of this happens depends on a person’s lifetime of infections and health, not just their birthday.20PubMed Central. Immunosenescence and COVID-19 Age itself changed how the immune system handled the virus, and healthy older adults still faced much higher risk than young adults.
Some people also carry a hidden weak spot. Their blood holds antibodies that disable interferons, the body’s first alarm signals against viruses.21PubMed. Autoantibodies neutralizing type I IFNs are present in ~4% of uninfected individuals over 70 years old and account for ~20% of COVID-19 deaths Interferons slow the virus in the first days, before targeted defenses arrive. Without them the virus multiplies unchecked, and a later, larger immune response then drives the dangerous inflammation described earlier.
These antibodies exist before infection and become sharply more common after age 70, found in about 4% of uninfected people over 70.22PubMed. Autoantibodies neutralizing type I IFNs are present in ~4% of uninfected individuals over 70 years old and account for ~20% of COVID-19 deaths They account for about 20% of COVID deaths and about 20% of critical cases in people over 80.23PubMed. Autoantibodies neutralizing type I IFNs are present in ~4% of uninfected individuals over 70 years old and account for ~20% of COVID-19 deaths Across younger and older severely ill patients, roughly 1 in 10 carried them.24PubMed Central. Type I interferon autoantibodies are associated with systemic immune alterations in patients with COVID-19
How Diabetes, Obesity and Other Conditions Raised the Risk
Obesity and diabetes raise the risk of infections, complications after infection, and death from serious infections, COVID included.25Wiley Online Library (Diabetes/Metabolism Research and Reviews). Obesity and diabetes as high-risk factors for severe coronavirus disease 2019 (Covid-19) Obesity also affects breathing: extra weight on the chest and belly makes deep breaths harder, and fat tissue itself produces inflammatory signals. It often comes with diabetes and high blood pressure, so the risks stack.
In the hospital, organ damage piled up. Kidney damage in COVID can come from the virus attacking kidney cells, tiny clots, and a struggling heart reducing blood flow.26PubMed Central. Acute Kidney Injury in COVID-19 Patients: Pathogenesis, Clinical Characteristics, Therapy, and Mortality Kidney injury tended to appear later, often as a knock-on effect of low oxygen, low blood pressure, and inflammation.27PubMed Central. Outcomes of Hospitalized Patients With COVID-19 With Acute Kidney Injury and Acute Cardiac Injury So patients with breathing failure or heart injury were more likely to develop kidney injury too.28PubMed Central. Clinical Features and Outcomes of COVID-19 Patients with Acute Kidney Injury and Acute Kidney Injury on Chronic Kidney Disease
Overwhelmed Hospitals and Hospital Infections
In the US, when ICU beds reached 75% full, an estimated 12,000 extra deaths followed.29PubMed Central. Impact of hospital strain on excess deaths during the COVID-19 pandemic—United States, july 2020–july 2021 When ICUs went over 100% full, about 80,000 extra deaths were expected nationally in the next 2 weeks.30PubMed Central. Impact of hospital strain on excess deaths during the COVID-19 pandemic—United States, july 2020–july 2021 Patients admitted to UK ICUs during extreme strain in the second wave had about 15% to 17% higher odds of dying.31PubMed Central. Does Unprecedented ICU Capacity Strain, As Experienced During the COVID-19 Pandemic, Impact Patient Outcome? Non-COVID ICU patients admitted during extreme strain had up to 30% higher odds of dying.32PubMed Central. Does Unprecedented ICU Capacity Strain, As Experienced During the COVID-19 Pandemic, Impact Patient Outcome? Stretched staff, more patients per nurse, and non-ICU staff filling in were linked to more complications such as bedsores and breathing tubes coming out.33PubMed Central. The impact of intensive care strain on patients’ outcomes—a multinational observational cohort (UNITE-COVID) study
Overload hurt people without COVID too. Heart attack and stroke patients waited longer for beds, and surgeries and cancer care were postponed. Some deaths counted in the toll were these indirect losses.
Long ventilator stays brought new infections. In one hospital, about 1 in 7 COVID patients developed a second bacterial or fungal infection.34PubMed Central. Secondary bacterial and fungal infections in COVID-19 patients About 1 in 5 patients with a secondary bacterial infection died of it.35PubMed Central. Secondary bacterial and fungal infections in COVID-19 patients In one group of ventilated COVID patients, every case of ventilator pneumonia involved bacteria, often drug-resistant, and 68% also involved fungi.36PubMed Central. Bacterial and fungal ventilator associated pneumonia in critically ill COVID-19 patients during the second wave
Timely treatment made a difference. The steroid dexamethasone lowered deaths in hospitalized COVID patients who needed oxygen.37PubMed. Dexamethasone in Hospitalized Patients with Covid-19 Steroids calm the runaway inflammation, so they help once the body’s reaction, not the virus, is the main threat. Given too early to mild cases they may not help. Starting the drug late, on the other hand, was linked to nearly double the odds of death in one hospital.38PubMed Central. Timing of Dexamethasone Initiation and Its Impact on the Outcome of COVID-19 Patients
How Many People COVID Really Killed
Researchers count excess deaths: how many more people died than a normal year would bring. The World Health Organization estimated 14.83 million excess deaths globally for 2020 and 2021, 2.74 times the 5.42 million reported.39PubMed Central. The WHO estimates of excess mortality associated with the COVID-19 pandemic Another major estimate put them at 18.2 million by the end of 2021, about 3 times the 5.94 million reported.40PubMed Central. Estimating excess mortality due to the COVID-19 pandemic: a systematic analysis of COVID-19-related mortality, 2020–21 The two big estimates differ because both models estimate deaths where data are missing, using different methods and assumptions. Undercounting was much larger in countries without complete death records.41eLife. Tracking excess mortality across countries during the COVID-19 pandemic with the World Mortality Dataset
Official counts missed many people for several reasons. Many died at home without a test, and some countries lack full death registration. Deaths above what a normal year would bring also include people who died because care was disrupted, and those never appear as COVID deaths.
Why Poverty, Race and Pollution Shaped Who Died
In the US, counties with more Black residents had higher COVID death rates.42PubMed Central. The Relationship Between Social Determinants of Health and Racial Disparities in COVID-19 Mortality But once poverty, housing, insurance, and other social conditions were accounted for, the link with race disappeared.43PubMed Central. The Relationship Between Social Determinants of Health and Racial Disparities in COVID-19 Mortality The gap came from circumstances, not genetic differences.44PubMed Central. The Relationship Between Social Determinants of Health and Racial Disparities in COVID-19 Mortality Essential-service jobs, poverty, and limited access to care raised exposure and risk.45PubMed Central. Racial, Economic, and Health Inequality and COVID-19 Infection in the United States Counties with more segregation, lower education, more uninsured people, and less remote work had more excess deaths.46PubMed. Racial, Socioeconomic, and Structural Determinants of Age-adjusted Excess COVID-19 Mortality Across U.S. Counties
Exposure and severity stacked up. Workers who couldn’t stay home met the virus more often, and crowded housing spread it through families, including elders. Less insurance meant later care, when treatment helps less.
Long-term air pollution added risk. In US counties, each small rise in long-term fine-particle pollution was linked to an 8% to 11% higher COVID death rate.47PubMed Central. Exposure to air pollution and COVID-19 mortality in the United States: A nationwide cross-sectional study48PubMed Central. Air pollution and COVID-19 mortality in the United States: Strengths and limitations of an ecological regression analysis In California, people in the most polluted areas had a 51% higher risk of dying of COVID than those in the cleanest.49PubMed Central. Association between long-term exposure to particulate air pollution with SARS-CoV-2 infections and COVID-19 deaths in California, U.S.A. Polluted areas are also often poorer, so the effects overlap, and these are area-level links, not proof for any one person.
How Silent Spread and Misinformation Fueled the Toll
Many infected people had no symptoms and kept going about their lives. A virus that’s milder per person but spreads silently can cause more total deaths through sheer numbers of infections.50PubMed Central. Intermediate levels of asymptomatic transmission can lead to the highest epidemic fatalities Isolating sick people doesn’t catch those who feel fine, and spread before symptoms started helped the virus outpace testing. Masks, distance, and ventilation mattered because no one knew who was infected.
Even a modest amount of misinformation can greatly increase spread and the severity of an outbreak.51PubMed Central. Assessing the impact of misinformation during the spread of infectious diseases It led people to ignore guidance and skip vaccination. Some turned to unproven treatments instead of seeking care. Distrust slowed responses when speed mattered most.
Why COVID Is Less Deadly Now
COVID vaccines reduced infections, severe illness, hospital stays, and deaths across many groups.52PubMed Central. The efficacy and effectiveness of the COVID-19 vaccines in reducing infection, severity, hospitalization, and mortality: a systematic review Among US nursing home residents during Delta, three doses gave about 89% protection against death.53Open Forum Infectious Diseases. Real-world Effectiveness of mRNA COVID-19 Vaccines Among US Nursing Home Residents Aged ≥65 Years in the Pre-Delta and High Delta Periods Two-dose protection fell from about 65% to 45% after six months.54Open Forum Infectious Diseases. Real-world Effectiveness of mRNA COVID-19 Vaccines Among US Nursing Home Residents Aged ≥65 Years in the Pre-Delta and High Delta Periods Protection against catching Omicron faded within about 100 days of a booster.55PubMed Central. Real-world COVID-19 vaccine effectiveness against the Omicron BA.2 variant in a SARS-CoV-2 infection-naive population Staying up to date matters most for older and high-risk people.
The virus itself also became milder. Omicron caused milder illness than Delta even in unvaccinated patients, pointing to a less harmful variant.56PubMed Central. Comparison of the clinical characteristics of SARS-CoV-2 Delta (B.1.617.2) and Omicron (B.1.1.529) infected patients from a single hospitalist service Hospitalized adults with Omicron had about 40% lower risk of dying within 30 days than those with Delta.57PubMed Central. Clinical progression, disease severity, and mortality among adults hospitalized with COVID-19 caused by the Omicron and Delta SARS-CoV-2 variants: A population-based, matched cohort study