Study findings updated . AI-assisted evidence summary, not individual medical advice or a clinician endorsement.
Key findings
Omicron: no extra clotsClot risk after COVID-19 matched other respiratory infections, May 2022 to Nov 2024
Since mid-2022, COVID-19's clot risk has matched other respiratory infections
Across the pandemic, US adults with COVID-19 had higher odds of a leg or lung clot within a month than adults with other respiratory infections (odds ratio 1.45). But in the Contemporary Omicron period, May 2022 to Nov 2024, risk was similar between the groups, especially among vaccinated people. Vaccination was linked to lower odds of clots (odds ratio 0.41).
The alarming clot risks reported early in the pandemic may not describe a typical COVID-19 infection today, especially in vaccinated people. That reshapes how worried most people should be about clots after a recent case. Observational health-record study that cannot prove cause; it compared COVID-19 with other respiratory infections rather than with uninfected people, and coded diagnoses can miss clots.
A retrospective study of US electronic health records from the N3C database, covering 756,217 adults with COVID-19 and 678,747 with other respiratory infections, March 2020 to November 2024.
From the study: “with similar risk between patients with COVID-19 and ARI in the Contemporary Omicron period (May 2022-Nov 2024), especially among vaccinated individuals.”
Omicron: no extra clots
Soff SP et al. A comprehensive assessment of the impact of COVID variant and vaccination on venous thromboembolism and mortality. Research and practice in thrombosis and haemostasis. 2026.
From the study: “Patients with COVID-19 had an increased risk of VTE (odds ratio [OR]: 1.45) and mortality (OR: 1.80) compared to the ARI cohort.”
Omicron: no extra clots
Soff SP et al. A comprehensive assessment of the impact of COVID variant and vaccination on venous thromboembolism and mortality. Research and practice in thrombosis and haemostasis. 2026.
From the study: “Vaccination was associated with a reduced odds of VTE (OR: 0.41) and mortality (OR: 0.41).”
Omicron: no extra clots
Soff SP et al. A comprehensive assessment of the impact of COVID variant and vaccination on venous thromboembolism and mortality. Research and practice in thrombosis and haemostasis. 2026.
1.6× flu's clot riskVein clots within 90 days of a COVID-19 vs flu hospital stay (hazard ratio)
Hospitalized COVID-19 patients had vein clots far more often than flu patients
Among US adults hospitalized with COVID-19 before vaccines were available, 9.5% were diagnosed with a vein clot in the leg or lung within 90 days, vs 5.3% of adults hospitalized with flu. After adjusting for differences, the clot hazard was 1.6 times flu's before vaccines and 1.9 times during the vaccine rollout. Heart attack and stroke risk did not differ significantly.
It suggests COVID-19 was not just another bad chest infection for the blood: patients sick enough for hospital faced clearly more leg and lung clots than flu patients, helping explain the focus on clot prevention in COVID-19 care. Insurance and health-system records from 2020 and 2021 that cannot prove cause; a later US study found COVID-19's clot risk matched other respiratory infections in the Omicron era.
A US FDA Sentinel study of health records for 85,637 adults hospitalized with COVID-19 in 2020–2021, compared with 8,269 adults hospitalized with flu in 2018–2019.
From the study: “The 90-day absolute risk of venous thromboembolism was 5.3% (95% CI, 4.9%-5.8%) in patients with influenza vs 9.5% (95% CI, 9.2%-9.7%) in patients with COVID-19 before vaccine availability”
1.6× flu's clot risk
Lo Re V et al. Association of COVID-19 vs Influenza With Risk of Arterial and Venous Thrombotic Events Among Hospitalized Patients. JAMA. 2022.
From the study: “the risk of venous thromboembolism was significantly higher among patients with COVID-19 before vaccine availability (adjusted HR, 1.60 [95% CI, 1.43-1.79])”
1.6× flu's clot risk
Lo Re V et al. Association of COVID-19 vs Influenza With Risk of Arterial and Venous Thrombotic Events Among Hospitalized Patients. JAMA. 2022.
From the study: “and during vaccine availability (adjusted HR, 1.89 [95% CI, 1.68-2.12])”
1.6× flu's clot risk
Lo Re V et al. Association of COVID-19 vs Influenza With Risk of Arterial and Venous Thrombotic Events Among Hospitalized Patients. JAMA. 2022.
49 weeks of higher riskVein clots stayed more common than in people without COVID-19, England and Wales
Vein clot risk stayed raised for nearly a year after COVID-19
In England and Wales, people diagnosed with COVID-19 had 33.2 times the hazard of a first vein clot in the week after diagnosis, compared with people without a COVID-19 diagnosis. The excess faded but lingered: 1.8 times in weeks 27 to 49. Even after COVID-19 that did not need a hospital stay, it was 1.77 times in weeks 27 to 49.
Clot risk does not end when the fever does. Warning signs of a leg or lung clot, such as a swollen, painful leg or sudden breathlessness, deserve attention for months after infection, even one that never needed a hospital. Observational data from 2020, before vaccines and Omicron, and week-1 figures may be inflated; a later US study found clot risk matched other respiratory infections in the Omicron era.
A population-wide study of linked health records for about 48 million adults in England and Wales, comparing clots after a COVID-19 diagnosis in 2020 with people without one.
From the study: “Adjusted hazard ratios for first VTE after COVID-19 diagnosis declined from 33.2 (95% CI, 31.3-35.2) in week 1 to 1.80 (95% CI, 1.50-2.17) during weeks 27 to 49.”
49 weeks of higher risk
Knight R et al. Association of COVID-19 With Major Arterial and Venous Thrombotic Diseases: A Population-Wide Cohort Study of 48 Million Adults in England and Wales. Circulation. 2022.
From the study: “After nonhospitalized COVID-19, aHRs were 2.56 (2.22–2.95), 2.22 (1.84–2.68), 1.98 (1.74–2.25), and 1.77 (1.38–2.27) during weeks 5 to 8, 9 to 12, 13 to 26, and 27 to 49, respectively.”
49 weeks of higher risk
Knight R et al. Association of COVID-19 With Major Arterial and Venous Thrombotic Diseases: A Population-Wide Cohort Study of 48 Million Adults in England and Wales. Circulation. 2022.
From the study: “incidence remains elevated up to 49 weeks after COVID-19 diagnosis.”
49 weeks of higher risk
Knight R et al. Association of COVID-19 With Major Arterial and Venous Thrombotic Diseases: A Population-Wide Cohort Study of 48 Million Adults in England and Wales. Circulation. 2022.
About 1 in 25 helpedGiven full-dose heparin per one more leaving hospital alive without ICU-level support
Full-dose heparin helped hospital patients who were not yet critically ill
In hospitalized COVID-19 patients who did not need ICU-level support, full-dose (therapeutic) heparin, a blood thinner, was compared with the usual lower preventive dose. 80.2% on full dose left hospital alive without heart or lung organ support, vs 76.4% on usual care, an adjusted gain of 4.0 percentage points. Major bleeding was 1.9% vs 0.9%.
It was among the first large trials to show a stronger blood-thinner dose could help some COVID-19 patients, while making the trade-off plain: a small rise in serious bleeding. Open-label trial from 2020 and 2021 that excluded people at high bleeding risk; it applies only to patients not yet critically ill, and a parallel trial found no benefit in ICU patients.
An international randomized trial (ATTACC, ACTIV-4a and REMAP-CAP) of 2,219 hospitalized, non-critically ill COVID-19 patients, about half enrolled in the US, in 2020 and 2021.
From the study: “The adjusted absolute between-group difference in survival until hospital discharge without organ support favoring therapeutic-dose anticoagulation was 4.0 percentage points”
About 1 in 25 helped
ATTACC Investigators et al. Therapeutic Anticoagulation with Heparin in Noncritically Ill Patients with Covid-19. The New England journal of medicine. 2021.
From the study: “801 (76.4%) survived until hospital discharge without receipt of organ support during the first 21 days, as compared with 939 of 1171 patients (80.2%)”
About 1 in 25 helped
ATTACC Investigators et al. Therapeutic Anticoagulation with Heparin in Noncritically Ill Patients with Covid-19. The New England journal of medicine. 2021.
From the study: “Major bleeding occurred in 1.9% of the patients receiving therapeutic-dose anticoagulation and in 0.9% of those receiving thromboprophylaxis.”
About 1 in 25 helped
ATTACC Investigators et al. Therapeutic Anticoagulation with Heparin in Noncritically Ill Patients with Covid-19. The New England journal of medicine. 2021.
10.6× hospitalizationsRecent blood-cancer treatment, 3+ vaccine doses, vs people without it (rate ratio)
Vaccinated blood-cancer patients still faced far higher COVID-19 hospitalization rates
In England in the Omicron era, among people with at least 3 vaccine doses, those recently treated for a blood cancer had 10.6 times the rate of COVID-19 hospital stays of people without that condition, after adjusting for age, sex and other illnesses. Overall, people with weakened immune systems were 3.9% of the population but 22% of COVID-19 hospitalizations.
Vaccines alone did not fully shield people getting blood-cancer treatment. The authors say such groups urgently need protection beyond current vaccination programs. Observational study of English health records funded by AstraZeneca; it shows higher rates, not their cause, and cannot fully separate the cancer from its treatment.
A retrospective study of health records for a random 25% sample of England's population aged 12 and over, about 12 million people, followed through 2022.
From the study: “and recent treatment for haematological malignancy (aIRR 10.6, 95% CI 9.5-11.9)”
10.6× hospitalizations
Evans RA et al. Impact of COVID-19 on immunocompromised populations during the Omicron era: insights from the observational population-based INFORM study. The Lancet regional health. Europe. 2023.
From the study: “Restricting to those vaccinated with ≥3 doses of COVID-19 vaccine (∼84% of immunocompromised and 51% of the general population)”
10.6× hospitalizations
Evans RA et al. Impact of COVID-19 on immunocompromised populations during the Omicron era: insights from the observational population-based INFORM study. The Lancet regional health. Europe. 2023.
From the study: “immunocompromised individuals accounted for 3.9% of the study population, but 22% (4585/20,910) of COVID-19 hospitalisations”
10.6× hospitalizations
Evans RA et al. Impact of COVID-19 on immunocompromised populations during the Omicron era: insights from the observational population-based INFORM study. The Lancet regional health. Europe. 2023.
3.8 per million dosesRate of the vaccine clotting syndrome (TTS) after the Johnson & Johnson shot, US
A rare clotting syndrome followed J&J shots, almost never mRNA shots
US surveillance found 3.83 reported cases of thrombosis with thrombocytopenia syndrome (clots plus low platelets) per million doses of the Johnson & Johnson vaccine (Ad26.COV2.S), vs 0.00855 per million mRNA doses. After J&J, patients were mostly women (69%), median age 44.5. Among those hospitalized after J&J, 15% died and 68% went home.
It puts the vaccine clot scare in proportion: a real but very rare danger tied to the J&J shot, while the handful of cases after mRNA shots looked like a normal background rate. When it did occur, it was often severe. Based on voluntary reports to a national safety system, so some cases may have been missed and follow-up was incomplete.
A CDC case series of 57 confirmed cases reported to the US Vaccine Adverse Event Reporting System after COVID-19 vaccination, December 2020 to August 2021.
From the study: “Reporting rates for TTS were 3.83 per million vaccine doses (Ad26.COV2.S) and 0.00855 per million vaccine doses (mRNA-based COVID-19 vaccines).”
3.8 per million doses
See I et al. Case Series of Thrombosis With Thrombocytopenia Syndrome After COVID-19 Vaccination-United States, December 2020 to August 2021. Annals of internal medicine. 2022.
From the study: “Outcomes of hospitalizations after Ad26.COV2.S vaccination included death (15%), discharge to postacute care (17%), and discharge home (68%).”
3.8 per million doses
See I et al. Case Series of Thrombosis With Thrombocytopenia Syndrome After COVID-19 Vaccination-United States, December 2020 to August 2021. Annals of internal medicine. 2022.
From the study: “The median age of patients with TTS after Ad26.COV2.S vaccination was 44.5 years (range, 18 to 70 years), and 69% of patients were women.”
3.8 per million doses
See I et al. Case Series of Thrombosis With Thrombocytopenia Syndrome After COVID-19 Vaccination-United States, December 2020 to August 2021. Annals of internal medicine. 2022.
About 1 in 17 helpedHigh-risk patients treated after discharge per one spared a clot or vascular death
Extended blood thinner after discharge cut clots in high-risk COVID-19 patients
High-risk COVID-19 patients leaving hospital in Brazil were randomly given either the blood thinner rivaroxaban or no blood thinner for 35 days. Clots (including some found on scans) or death from heart or vessel causes occurred in 3% on the drug vs 9% with no blood thinner. No major bleeding occurred in either group.
The weeks after a COVID-19 hospital stay are a risky time for clots. This was the first randomized trial to show a clinical benefit from clot prevention after discharge, in carefully chosen high-risk patients; larger trials were planned to confirm it. Small (320 patients), open-label trial in Brazil funded by Bayer; results need confirming in larger trials.
The MICHELLE trial randomized 320 high-risk patients discharged after a COVID-19 hospital stay at 14 centers in Brazil, October 2020 to June 2021.
From the study: “The primary efficacy outcome occurred in five (3%) of 159 patients assigned to rivaroxaban and 15 (9%) of 159 patients assigned to no anticoagulation”
About 1 in 17 helped
Ramacciotti E et al. Rivaroxaban versus no anticoagulation for post-discharge thromboprophylaxis after hospitalisation for COVID-19 (MICHELLE): an open-label, multicentre, randomised, controlled trial. Lancet (London, England). 2022.
From the study: “No major bleeding occurred in either study group.”
About 1 in 17 helped
Ramacciotti E et al. Rivaroxaban versus no anticoagulation for post-discharge thromboprophylaxis after hospitalisation for COVID-19 (MICHELLE): an open-label, multicentre, randomised, controlled trial. Lancet (London, England). 2022.
From the study: “were randomly assigned (1:1) to receive, at hospital discharge, rivaroxaban 10 mg/day or no anticoagulation for 35 days”
About 1 in 17 helped
Ramacciotti E et al. Rivaroxaban versus no anticoagulation for post-discharge thromboprophylaxis after hospitalisation for COVID-19 (MICHELLE): an open-label, multicentre, randomised, controlled trial. Lancet (London, England). 2022.
12 weeks, no benefitBlood thinner rivaroxaban vs no drug for long COVID fatigue in UK adults
A blood thinner did not ease long COVID fatigue, but many improved
In UK adults with long COVID who had not been hospitalized, 12 weeks of the blood thinner rivaroxaban did not significantly lower fatigue scores compared with specialist care and no study drug (1.06 points lower, within chance). Across all groups, including no drug, average fatigue scores fell by 4.3 points, from 36.8 to 32.5, over 12 weeks.
Blood thinners have been suggested for long COVID, partly on a theory that tiny clots drive symptoms. In this trial rivaroxaban gave no clear fatigue benefit, while fatigue eased for many people under specialist care alone. Open-label trial, so people knew what they took, with self-reported fatigue; it tested one drug at one dose and cannot rule out benefit in specific subgroups.
A randomized trial at 12 UK NHS long COVID clinics, 2022 to 2024, giving 778 non-hospitalized adults colchicine, rivaroxaban, famotidine-loratadine or no drug for 12 weeks.
From the study: “but not in the rivaroxaban group (-1·06 points [-2·47 to 0·35, p=0·139), compared with no study drug at 12 weeks.”
12 weeks, no benefit
STIMULATE-ICP consortium. Efficacy and safety of rivaroxaban, colchicine, and famotidine-loratadine with specialist supportive clinical care for fatigue in patients with post-COVID-19 condition in the UK: a multisite, open-label, randomised controlled trial. The Lancet. Infectious diseases. 2026.
From the study: “Across all trial groups, there was severe baseline fatigue (mean FAS score 36·8 [SD 7·49]) and a clinically relevant mean FAS reduction of 4·3 points to 32·5 (SD 9·13) at 12 weeks.”
12 weeks, no benefit
STIMULATE-ICP consortium. Efficacy and safety of rivaroxaban, colchicine, and famotidine-loratadine with specialist supportive clinical care for fatigue in patients with post-COVID-19 condition in the UK: a multisite, open-label, randomised controlled trial. The Lancet. Infectious diseases. 2026.
Study-specific findings. Different populations, treatments and follow-up periods can produce different results.
No clear difference was established for the combined death or clot outcome
No clear difference
In hospitalized patients with COVID-19, therapeutic versus prophylactic anticoagulation did not establish a difference in the combined death or blood-clot outcome through 42 days.
The outcome included death from any cause or clots in veins or arteries. The interval included no difference; the per-protocol analysis was also uncertain. Bleeding events were 7 with therapeutic and 6 with prophylactic anticoagulation, without a comparative effect estimate.
Study details & original results
Karakas, Mahir et al.. Prophylactic vs Therapeutic Anticoagulation in Hospitalized Patients With COVID-19: A Randomized Clinical Trial.. JAMA network open. 2026.
Time to first all-cause death, venous thromboembolism, or arterial thromboembolism through 42 days with therapeutic vs prophylactic anticoagulation; not statistically significant (P = .40), with primary endpoint assessment unavailable for 2 patients.
HR 0.74 95% CI: 0.38-1.48
Per-protocol analysis of the primary composite outcome; not statistically significant (P = .26).
HR 0.66 95% CI: 0.32-1.37
Reported bleeding-event counts in the therapeutic-anticoagulation intervention group vs prophylactic-anticoagulation control group; no comparative effect estimate was provided.
7 vs 6 bleeding events
The reported result does not establish a difference; it does not prove equivalence.
Nebulized heparin findings leave survival and bleeding interpretation bounded
A meta-analysis of randomized trials in adults with acute respiratory failure reported a lower mortality estimate with nebulized heparin; the mortality comparison group is not available in this summary.
For nebulized heparin versus control, major bleeding showed no clear difference and its interval included no difference. The wide interval does not establish equal bleeding risk or safety. These are acute respiratory-failure findings, not long COVID treatment evidence.
Study details & original results
Fresilli, Stefano et al.. Nebulized Heparin in Adults With Acute Respiratory Failure: A Meta-Analysis of Randomized Trials.. Critical care medicine. 2026.
All-cause mortality reduction with nebulized heparin in respiratory failure
RR 0.79 95% CI: 0.66-0.95
Major bleeding with nebulized heparin vs control (non-significant)
Dates show when findings were added here, not when papers were published. Study populations and comparisons differ.
Browse all 5 recorded findingsSearch the original results, comparisons and study details.
2 of 2 source groupsFindings stay together with their study.
SOURCE 033 findings
Karakas, Mahir et al.. Prophylactic vs Therapeutic Anticoagulation in Hospitalized Patients With COVID-19: A Randomized Clinical Trial.. JAMA network open. 2026.
Time to first all-cause death, venous thromboembolism, or arterial thromboembolism through 42 days with therapeutic vs prophylactic anticoagulation; not statistically significant (P = .40), with primary endpoint assessment unavailable for 2 patients.
Reported bleeding-event counts in the therapeutic-anticoagulation intervention group vs prophylactic-anticoagulation control group; no comparative effect estimate was provided.
Fresilli, Stefano et al.. Nebulized Heparin in Adults With Acute Respiratory Failure: A Meta-Analysis of Randomized Trials.. Critical care medicine. 2026.
Browse the 11 original sourcesThe complete bibliography behind this monitor.
[1]
Fresilli, Stefano et al.. Nebulized Heparin in Adults With Acute Respiratory Failure: A Meta-Analysis of Randomized Trials.. Critical care medicine. 2026.
Bradbury, Charlotte A et al.. Intermediate dose heparin thromboprophylaxis among critically ill patients with COVID-19: a randomized clinical trial.. Journal of thrombosis and haemostasis : JTH. 2026.
Karakas, Mahir et al.. Prophylactic vs Therapeutic Anticoagulation in Hospitalized Patients With COVID-19: A Randomized Clinical Trial.. JAMA network open. 2026.
Soff SP et al. A comprehensive assessment of the impact of COVID variant and vaccination on venous thromboembolism and mortality. Research and practice in thrombosis and haemostasis. 2026.
Knight R et al. Association of COVID-19 With Major Arterial and Venous Thrombotic Diseases: A Population-Wide Cohort Study of 48 Million Adults in England and Wales. Circulation. 2022.
ATTACC Investigators et al. Therapeutic Anticoagulation with Heparin in Noncritically Ill Patients with Covid-19. The New England journal of medicine. 2021.
Evans RA et al. Impact of COVID-19 on immunocompromised populations during the Omicron era: insights from the observational population-based INFORM study. The Lancet regional health. Europe. 2023.
See I et al. Case Series of Thrombosis With Thrombocytopenia Syndrome After COVID-19 Vaccination-United States, December 2020 to August 2021. Annals of internal medicine. 2022.
Ramacciotti E et al. Rivaroxaban versus no anticoagulation for post-discharge thromboprophylaxis after hospitalisation for COVID-19 (MICHELLE): an open-label, multicentre, randomised, controlled trial. Lancet (London, England). 2022.
STIMULATE-ICP consortium. Efficacy and safety of rivaroxaban, colchicine, and famotidine-loratadine with specialist supportive clinical care for fatigue in patients with post-COVID-19 condition in the UK: a multisite, open-label, randomised controlled trial. The Lancet. Infectious diseases. 2026.
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