SARS-CoV-2 damages the lungs through a combination of direct viral attack on key lung cells and a sometimes-destructive immune overreaction that can continue injuring tissue even after the virus itself is waning. The resulting harm ranges from temporary inflammation that clears within weeks to lasting scarring visible on imaging years later. How much damage occurs, and how well the lungs bounce back, depends on disease severity, individual biology, and the variant involved.
How the Virus Gets Into Lung Tissue
The trouble begins with a specific molecular handshake. In the lower airways, SARS-CoV-2 latches onto a receptor called ACE2, using an enzyme on the cell surface to gain entry. The cells most vulnerable to this are the type II alveolar cells that line the tiny air sacs where oxygen passes into the blood.1PubMed Central. Alveolar type II cells and pulmonary surfactant in COVID-19 era These cells have a dual role: they help produce surfactant, the slippery coating that keeps air sacs from collapsing, and they serve as stem-like precursors that regenerate damaged lung lining. When the virus destroys them, it knocks out both of those functions at once.
Once inside, the virus hijacks the cell’s machinery to replicate, killing the host cell in the process. That cell death sends distress signals that attract immune cells. In a well-calibrated response, the immune system clears the virus and the lungs heal. In a poorly calibrated one, the immune response spirals into something far more destructive than the virus alone.
The Immune Overreaction That Drives the Worst Damage
The term “cytokine storm” became familiar during the pandemic for good reason. In severe COVID, the body releases a flood of inflammatory signaling molecules. This hyperinflammatory state injures the very cells the immune system is supposed to protect, damaging both the lining of the air sacs and the walls of surrounding blood vessels.2PubMed Central. Lung under attack by COVID-19-induced cytokine storm: pathogenic mechanisms and therapeutic implications Neutrophils and macrophages, normally helpful immune cells, flood into the lung tissue in excessive numbers and release their own destructive chemicals.
One key driver of this cascade involves the signaling molecule IL-6. Overactivation of the IL-6 pathway triggers the release of additional inflammatory signals and a substance that increases blood-vessel leakiness. At the same time, it reduces a protein that normally helps keep vessel walls sealed. The result is fluid pouring into lung tissue where it doesn’t belong, contributing to the waterlogged, stiff lungs that characterize severe COVID pneumonia.3Signal Transduction and Targeted Therapy. The signal pathways and treatment of cytokine storm in COVID-19
Blood Vessel Damage and Microclots
COVID lung injury is not purely an airway disease. The virus and the immune response together damage the endothelial cells lining blood vessels within the lungs. This endothelial injury triggers the formation of tiny clots, called microthrombi, throughout the lung’s capillary network.4PubMed Central. Damage to endothelial barriers and its contribution to long COVID Autopsy studies documented this progression clearly: early disease showed inflammation of tiny blood vessels with microthrombosis, while later stages revealed ongoing clotting in small-to-medium pulmonary vessels, sometimes causing localized areas resembling tissue death from blocked blood flow.5PubMed Central. The evolution of pulmonary pathology in fatal COVID-19 disease: an autopsy study with clinical correlation
This vascular component helps explain why some patients with COVID developed dangerously low oxygen levels even when their lungs didn’t appear as severely affected on imaging as expected. Clots blocking blood flow through the lungs meant that even well-ventilated air sacs couldn’t efficiently transfer oxygen into the bloodstream.
The Phases of Lung Injury
When researchers examined lung tissue from patients who died of COVID, a clear timeline emerged. The dominant pattern was diffuse alveolar damage, found in roughly 87% of autopsy cases in one large multi-institutional study.6PubMed Central. COVID-19 pulmonary pathology: a multi-institutional autopsy cohort from Italy and New York City This damage unfolded in stages:
- Exudative phase (week one): Fluid and protein leak into the air sacs, forming glassy membranes along their walls. This is the stage associated with acute respiratory distress.
- Proliferative/organizing phase (weeks two to three): The body begins attempting repair. Immune cells accumulate, and fibroblasts start laying down early scar tissue within the air sacs.
- Fibrotic phase (after week three): In patients whose disease persists, the repair process overshoots, and dense fibrous scar tissue replaces normal lung architecture.7PubMed Central. Histopathological features in fatal COVID-19 acute respiratory distress syndrome
A distinctive finding in COVID specifically was an unusual pattern of immune-cell accumulation that appeared at the boundary between the exudative and proliferative phases, most prominently around the fourth week of illness.8Human Pathology. Distinctive pseudopalisaded histiocytic hyperplasia characterizes the transition of exudative to proliferative phase of diffuse alveolar damage in patients dying of COVID-19 This finding distinguished COVID-related lung damage from similar patterns caused by other infections or by mechanical ventilation.
What CT Scans Reveal
Chest imaging became a crucial diagnostic and monitoring tool during the pandemic. The hallmark appearance of COVID pneumonia on CT is ground-glass opacities, hazy patches that look like frosted glass, typically appearing on both sides of the lungs and concentrated toward the back and outer edges.9PubMed Central. Chest CT features of coronavirus disease 2019 (COVID-19) pneumonia: key points for radiologists As the disease progresses, these hazy patches can solidify into denser consolidation, and additional patterns appear including linear streaks, a “crazy-paving” pattern resembling cracked pavement, and a “reversed halo” sign where a ring of denser tissue surrounds a central ground-glass area.10PubMed Central. Chest CT Findings in Coronavirus Disease-19 (COVID-19): Relationship to Duration of Infection
What makes follow-up imaging so informative is that these patterns evolve over time, and not all of them resolve. Studies of patients hospitalized during the first year of the pandemic found that roughly a quarter to over half still had visible abnormalities on CT more than a year after infection, including ground-glass opacity, scarring patterns, widened airways, and air trapping.11PubMed Central. Chronic Lung Injury after COVID-19 Pneumonia: Clinical, Radiologic, and Histopathologic Perspectives
Fibrosis and the Scarring Question
The possibility of permanent lung scarring was among the most concerning long-term outcomes of COVID. The virus promotes fibrosis through a specific biological pathway: infected alveolar cells ramp up production of a growth factor called TGF-β, which drives a process where lung-lining cells transform into scar-producing cells. This leads to collagen accumulation and stiffening of lung tissue.12Frontiers in Pharmacology. Molecular mechanisms of COVID-19-induced pulmonary fibrosis and epithelial-mesenchymal transition
A systematic review and meta-analysis tracking CT findings over time found that fibrotic-like changes were the most common persistent abnormality, present in about 44% of patients at short-term follow-up (one to six months) and 38% at long-term follow-up (one to two years). While overall lung abnormalities decreased over time, fibrotic-like changes themselves did not show a statistically significant decline. A pattern called honeycombing, representing advanced scarring, actually increased over time.13PubMed Central. Short- and Long-Term Chest-CT Findings after Recovery from COVID-19: A Systematic Review and Meta-Analysis Patients who had severe disease fared worse, with significantly higher rates of persistent abnormalities, scarring, and widened airways at long-term follow-up compared to those with milder illness.
How Lung Function Recovers Over Time
Imaging abnormalities tell only part of the story. Breathing tests provide a more functional picture, and the news here is somewhat more encouraging, at least for most patients. A longitudinal study following COVID survivors for three years after hospitalization found that the proportion with residual lung abnormalities on CT gradually dropped from 46% at six months to 36% at three years. The share of patients with reduced gas-transfer capacity, a measure of how well oxygen crosses from the air sacs into the blood, decreased from 49% at six months to 38% at three years. Walking distance on a standardized test also gradually improved.14PubMed. Long-term radiological and pulmonary function abnormalities at 3 years after COVID-19 hospitalisation: a longitudinal cohort study
But recovery wasn’t a straight upward line for everyone. A two-year study of lung-function trajectories found that while one key measure of lung volume increased slightly during the first year, it then declined during the second year. Total lung capacity and gas-transfer capacity both showed continuous declines from six months onward.15EClinicalMedicine. Lung-function trajectories in COVID-19 survivors after discharge: A two-year longitudinal cohort study This suggests that for a subset of patients, the lungs may not simply heal and stabilize but instead enter a slow decline reminiscent of progressive fibrotic lung disease.
Small Airways and the Breathing Difficulties That Imaging Misses
Some patients have persistent breathlessness that seems out of proportion to what their CT scans show. One explanation is damage to the smallest airways, structures too tiny to appear clearly on standard imaging. Case reports tracking patients for up to three years have documented persistent air trapping and abnormal small-airway function even as other lung findings improved, suggesting that COVID can cause a form of small airway disease that conventional tests don’t capture well.16PubMed Central. Small airway disease in post-acute COVID-19 syndrome, a non-conventional approach in three years follow-up of a patient with long COVID: a case report
Another underappreciated contributor is damage to the diaphragm itself. A study measuring diaphragm strength 15 months after hospitalization found that previously hospitalized COVID patients had significantly weaker diaphragms than healthy controls, regardless of whether they had been on a ventilator. The degree of diaphragm weakness correlated with the severity of breathlessness on exertion.17American Journal of Respiratory and Critical Care Medicine. Diaphragm Muscle Weakness Might Explain Exertional Dyspnea 15 Months after Hospitalization for COVID-19 In rare cases, COVID has even caused full diaphragm paralysis, leaving patients with chronic breathlessness months after their lung infection resolved.18PubMed Central. A Bilateral Diaphragmatic Paralysis Post-COVID-19 Infection: A Case Report and a Review of the Literature
Testing that goes beyond standard lung-function measures adds another layer. A study comparing long COVID patients to non-symptomatic survivors found significantly lower gas-transfer capacity and pulmonary capillary blood volume in the long COVID group, pointing to lasting damage in the lung’s vascular network even when airway function appeared relatively intact.19PubMed Central. Interrogating pulmonary diffusing capacity in long COVID: insights from DLCO and DLNO testing
Who Is Most Vulnerable to Lasting Damage
Pre-existing lung disease is an intuitive risk factor, and the data largely bear it out, though with some surprises. A large UK population study found that COPD was associated with a roughly 54% increase in the risk of COVID death, while pre-existing pulmonary fibrosis and other interstitial lung diseases also carried elevated risk.20PubMed Central. Association between pre-existing respiratory disease and its treatment, and severe COVID-19: a population cohort study A meta-analysis confirmed that COPD was clearly associated with more severe COVID, with the odds of severe disease roughly two and a half times higher in COPD patients.21PubMed Central. Effect of comorbid pulmonary disease on the severity of COVID-19: A systematic review and meta-analysis
Asthma, by contrast, showed a surprisingly muted association. While it was linked to a modestly higher risk of hospitalization, there was no clear evidence that asthma increased the risk of COVID death, even in patients with severe asthma on multiple medications.20PubMed Central. Association between pre-existing respiratory disease and its treatment, and severe COVID-19: a population cohort study This was one of the more counterintuitive findings of the pandemic and may relate to differences in the type of inflammation predominant in asthma versus the type driven by COVID.
Another individual vulnerability that gained attention was the presence of autoantibodies against the body’s own interferon, a key antiviral defense molecule. A meta-analysis of over 13,000 patients found these autoantibodies in roughly 18% of those tested, and their presence was dramatically more common in patients with critical or severe disease. Patients with neutralizing autoantibodies against one interferon subtype had more than 17 times the odds of severe illness compared to those without. These autoantibodies were also more common in men, which may partly explain the higher rates of severe COVID observed in male patients.22PubMed Central. Role of autoantibodies targeting interferon type 1 in COVID-19 severity: A systematic review and meta-analysis
Ventilator-Related Injury on Top of Viral Injury
For patients who required mechanical ventilation, there was a compounding problem: the ventilator itself could add to the lung damage. COVID patients often had a very strong drive to breathe, and when combined with the already-weakened state of their air sacs, this could lead to barotrauma, injury caused by pressure. Case reports documented pneumothorax (collapsed lung), air leaking into the chest cavity and surrounding tissues, even in patients whose ventilator settings were carefully managed to minimize pressure.23PubMed Central. Covid-19 Pneumonia and Ventilation-induced Lung Injury: A Case Report The combination of diffuse alveolar injury and high respiratory drive appeared to make COVID lungs particularly fragile and prone to these pressure-related complications.
How Variants Changed the Picture
Not all versions of SARS-CoV-2 hit the lungs equally hard. The shift from Delta to Omicron brought a measurable change in lung damage severity. Clinical comparisons found that Delta patients had significantly lower oxygen levels and markedly higher levels of inflammation markers and tissue-damage indicators compared to Omicron patients.24PubMed Central. Inflammatory tropism in COVID-19: a comparative analysis of Delta and Omicron variants Animal studies echoed this, showing higher viral loads in the lungs and greater visible lung damage with immune-cell infiltration from Delta compared to Omicron.25PubMed Central. Differential immunometabolic responses to Delta and Omicron SARS-CoV-2 variants in golden syrian hamsters
In intensive care, the difference was stark: adults admitted with Delta-era COVID nearly universally presented with severe respiratory distress, while the pattern of organ involvement shifted with later variants and in younger patients.26PubMed Central. The different manifestations of COVID-19 in adults and children: a cohort study in an intensive care unit This shift toward less lung-destructive variants is one reason hospitalizations and deaths dropped in later pandemic waves, though it was not the only factor.
Vaccination and Its Effect on Lung Injury
Vaccination substantially reduced the severity of lung damage when breakthrough infections occurred. An autopsy study comparing vaccinated and unvaccinated patients who died of COVID found a dramatic difference in lung involvement. Vaccinated patients were far more likely to have minimal or mild lung inflammation and far less likely to have the most severe category of pneumonia. Among vaccinated patients, about 21% had severe pneumonia scores compared to 55% of unvaccinated patients.27PubMed Central. Lung damage in SARS-CoV-2 patients: An autopsy study in the era of vaccination
Studies of living patients painted a similar picture. Vaccinated individuals had significantly lower rates of severe pulmonary involvement, with about 24% experiencing severe lung involvement compared to roughly 36% of unvaccinated patients.28PubMed Central. Impact of Vaccination Status on COVID-19 Severity and Pulmonary Involvement And the protective effect scaled with the severity of lung injury being prevented: vaccines were more effective at blocking extensive lung damage than at preventing any lung involvement at all.29medRxiv. Vaccine Effectiveness against Referral to Hospital and Severe Lung Injury Associated with COVID-19: A Case-Control Study in St. Petersburg, Russia
Treatment and Rehabilitation
For patients dealing with persistent lung damage, pulmonary rehabilitation has shown the most consistent evidence of benefit. A meta-analysis of randomized controlled trials found that structured rehab programs improved physical capacity, lung function, quality of life, fatigue, breathlessness, and anxiety in long COVID patients, though not depression.30PubMed Central. Effect of pulmonary rehabilitation for patients with long COVID-19: a systematic review and meta-analysis of randomized controlled trials However, not everyone responds equally. A study tracking patients nearly three years after discharge found that those with persistent breathing inefficiency during exercise did not show the same improvements in gas-transfer capacity or oxygen use that recovering patients did, suggesting a subgroup with more entrenched damage.31PubMed Central. Persisting exercise ventilatory inefficiency in subjects recovering from COVID-19. Longitudinal data analysis 34 months post-discharge
The pharmacological picture is murkier. Anti-fibrotic drugs like nintedanib and pirfenidone, already used for other forms of pulmonary fibrosis, have generated interest as potential treatments for post-COVID scarring. Early evidence suggests they may help reduce fibrotic changes and improve lung function, but their practical use is currently based more on theory and clinical experience than on robust trial data.32International Journal of Infectious Diseases. Impact of anti-fibrotic medications on post-COVID-19 pulmonary fibrosis: A systematic review and meta-analysis A complicating factor is that some post-COVID fibrosis may resolve on its own over time, making it difficult to determine whether improvement is due to treatment or natural healing.33Frontiers in Pharmacology. Pharmacological approaches to pulmonary fibrosis following COVID-19 Multiple clinical trials are underway or in design, but the field still lacks the kind of large, well-controlled studies needed to give definitive treatment recommendations for post-COVID lung scarring.