Published Jul 02, 2026 | 12:34 PM ⚊ Updated Jul 02, 2026 | 12:34 PM
Vaccination showed no association with thrombotic events (blood clots).
Synopsis: A nationwide ICMR study has found that smoking, existing illnesses, family history, previous blood clots and severe COVID-19, not COVID-19 vaccination, are the main drivers of heart attacks and thrombotic events among young Indians. Researchers say prevention should focus on controlling conventional cardiovascular risk factors and monitoring high-risk individuals.
For years, doctors and families have wrestled with the same question: why do seemingly healthy young adults suddenly collapse from heart attacks? Since the pandemic, COVID-19 vaccines have often been at the centre of that suspicion.
A new study led by the Indian Council of Medical Research (ICMR) suggests that vaccination has nothing to do with these events. Instead, they are associated with smoking, underlying medical conditions, a personal or family history of blood clots, and severe COVID-19.
Published in the Indian Journal of Medical Research, the study drew on data from 25 hospitals across India. Rather than relying solely on medical records, the researchers also interviewed patients directly.
“The occurrence of any arterial or venous thrombotic event was associated with the presence of comorbidity, previous personal or family history of a thrombotic event, and smoking,” the authors state.
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Soon after the pandemic, a string of cases of young people dying from heart attacks drew national attention. In the absence of a clear explanation, COVID-19 vaccines became an easy target.
Since 2021, India has administered more than two billion vaccine doses. It was crucial to find an answer grounded in evidence.
“Considering reports of myocardial infarction and other thrombotic events in apparently healthy individuals, this study was conducted to determine the association between COVID-19 vaccination, lifestyle, medical risk factors, and thrombotic events among young adults in India,” the paper notes.
A thrombotic event occurs when a blood clot forms inside a blood vessel and blocks blood flow. Depending on where it forms, it can trigger a heart attack or stroke, or block veins elsewhere in the body.
The researchers screened patients aged 18 to 45 who arrived at hospital with a new clotting event. They then identified patients admitted around the same time for unrelated reasons and matched them by hospital and admission date.
This study design allowed the researchers to compare the medical histories of people who developed clots with those who did not, while keeping the timing and setting similar. Differences between the two groups could then point to potential risk factors.
The study enrolled 767 patients with a thrombotic event and matched them with 2,144 controls. Among them, 432 patients had suffered a heart attack, matched against 1,293 controls.
Heart attacks accounted for 56.3 percent of all thrombotic events recorded in the study, followed by strokes at 25.8 percent. Clots in the veins draining the brain, the limbs and elsewhere in the body accounted for the remainder.
The researchers also examined the timing of vaccination. On average, patients were hospitalised roughly six months after vaccination, placing most cases well beyond the short period during which vaccine-associated clotting has been reported.
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The researchers examined vaccination status against both outcomes and analysed Covishield and Covaxin separately.
After adjusting for other risk factors, patients who had received two or more vaccine doses had virtually the same odds of a heart attack as unvaccinated patients. The same was true for thrombotic events overall. Analysed separately, Covishield and Covaxin produced comparable results.
The study also examined vaccine-induced immune thrombotic thrombocytopenia, a rare condition in which the immune system generates antibodies that activate platelets and trigger clotting, typically within four weeks of the first vaccine dose. Cases have been reported internationally following adenovirus-vector vaccines, although they have been far more common outside India.
“Although rare vaccine-induced immune thrombotic thrombocytopenia has been reported internationally following adenovirus-vector vaccines, particularly within four to 30 days after the first dose, the present study did not identify evidence linking COVID-19 vaccination to increased thrombotic events in young Indian adults,” the authors write.
Smoking produced the strongest signal in the data. Patients who had ever smoked had three times the odds of experiencing a thrombotic event than those who had never smoked, after the researchers adjusted for other risk factors. Among patients with heart attacks, the odds rose to 3.5 times.
Around 40 percent of patients with a thrombotic event reported a history of smoking, compared with 20 percent of the control group.
“Recent studies conducted in India on young adults determined smoking as a major risk factor for premature coronary artery disease and young stroke,” the authors write, adding that their findings “closely mirror” this pattern.
Unlike genetics or a history of illness, smoking is a modifiable risk factor, making it a direct target for public health intervention.
Patients with conditions such as high blood pressure, diabetes, kidney disease, high cholesterol, asthma, cancer, liver disease or thyroid disorders had four times the odds of a thrombotic event. Among heart attack patients, the odds increased to 4.6 times.
A previous thrombotic event outweighed every other risk factor measured. Patients with a prior clot had 30 times the odds of suffering another and 60 times the odds of a heart attack. A family history of clotting disorders independently tripled the odds.
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Vaccination showed no association with thrombotic events. Severe COVID-19 did.
Patients who had previously been hospitalised with COVID-19 had around three times the odds of a later thrombotic event than those who had not experienced severe infection.
The biological explanation is well established. Severe COVID-19 triggers an intense inflammatory response that damages the lining of blood vessels and makes blood more prone to clotting. These changes can persist for weeks after recovery.
“This finding is consistent with international evidence showing that severe SARS-CoV-2 infection triggers prolonged inflammation, endothelial damage and increased blood clotting risk,” the authors note.
The findings also sit within a broader pattern. Young Indians carry a higher burden of thrombotic disease than their Western counterparts, a difference researchers attribute to genetic susceptibility and elevated lipoprotein levels, alongside established risk factors such as smoking and poor diet.
Hospital data cited in the paper show that young patients account for 10 to 12 percent of heart attack admissions in India, compared with roughly 8 percent in Europe and North America. One hospital-based study found that young adults accounted for 31.3 percent of stroke admissions, a figure researchers attribute partly to referral patterns and partly to a genuine rise in thrombotic events among younger patients.
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The authors conclude by reaffirming the benefits of vaccination. They cite a review covering 5.6 crore participants that estimated vaccine effectiveness at 70.7 percent against infection and 87.4 percent against severe disease. Vaccinated people also had a 29 percent lower risk of developing long COVID.
A separate cohort study from Hong Kong reported lower rates of cardiovascular disease and death among people who completed the primary vaccination series or received a booster dose.
“We emphasise on the clear benefits of vaccination, demonstrated by earlier studies, in preventing severe morbidity and mortality from COVID-19,” the authors write.
The researchers set out to enrol a larger sample than they ultimately achieved. They were unable to trace 27 percent of both patients and controls, while some of those they did contact declined to participate.
Self-reported vaccination histories among the controls introduce further uncertainty, since participants recalling dates and doses from memory may misremember details. The researchers also used hospital admission as a proxy for severe COVID-19, an approach that may have underestimated severe illness during India’s second wave, when many patients who required hospital care could not obtain it.
The authors acknowledge that residual confounding from undetected prior infection cannot be ruled out entirely. They argue, however, that this risk is limited because exposure to SARS-CoV-2 was nearly universal during the study period.
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The findings point to familiar priorities rather than new ones. Smoking cessation programmes, earlier screening for chronic disease, and routine cardiovascular assessment for young adults with a personal or family history of thrombotic events are all interventions that existing health systems can expand.
Patients recovering from severe COVID-19 may also benefit from continued monitoring, given the extended period during which persistent inflammation and vascular damage appear to increase the risk of clotting.
The authors conclude by urging policymakers to pursue both goals at once: maintain high vaccination coverage while strengthening programmes that identify and manage the established risk factors behind heart attacks and thrombotic events among young Indians.
