Nikhil Prasad Fact checked by:Thailand Medical News Team Aug 26, 2026 50 minutes ago
Higher levels of COVID-19 circulating in communities may be followed by a measurable rise in serious blood clots in the lungs, with the strongest increase appearing about three weeks later, according to a large population study from Ontario, Canada.
COVID-19 activity was associated with increased pulmonary embolism hospitalizations, with the strongest risk
appearing around three weeks later
The findings provide new detail about when pulmonary embolism risk may rise following waves of SARS-CoV-2 activity. A pulmonary embolism, or PE, occurs when a blood clot blocks an artery in the lungs. It can become life-threatening and requires urgent medical treatment.
More Than 70,000 Cases Examined
The study analyzed hospital records from Ontario between 2011 and 2024. Of 85,803 hospitalizations involving pulmonary embolism, 70,599 first-time PE admissions were included in the final analysis.
Scientists compared weekly PE hospitalizations with activity from SARS-CoV-2, influenza A, influenza B and respiratory syncytial virus, commonly called RSV. They examined whether changes in viral circulation were followed by changes in PE hospitalizations over five weeks.
The researchers were from the Dalla Lana School of Public Health at the University of Toronto, Canada.
Risk Rose Most Clearly Around Week Three
The strongest finding involved SARS-CoV-2. Across the five-week period, greater COVID-19 activity was associated with a 12% increase in cumulative PE risk for each standard-deviation increase in viral activity.
The timing was particularly important. Risk was not significantly elevated during week one, but rose during week two. It reached its highest point during week three, when relative risk was 1.22, representing a 22% increase, before falling to a 9% increase during week four.
When investigators focused specifically on the pandemic years from 2020 through 2024 and adjusted for influenza A and RSV, the cumulative relative risk associated with SARS-CoV-2 was even higher at 1.26, corresponding to a 26% increase.
As this
Thailand Medical News report highlights, the delayed pattern may help explain how COVID-19 can continue affecting blood vessels after the initial stage of infection has passed.
Why COVID-19 May Trigger Clotting
SARS-CoV-2 can injure cells lining blood vessels, activate platelets and promote inflammation and clotting. The study suggests these processes may continue after viral replication begins declining, creating a longer “thrombo-inflammatory” period during which dangerous clots can develop.
A separate analysis of deep vein thrombosis showed a similar pattern, with risk peaking around week three, although the association was weaker than for pulmonary embolism.
The researchers cautioned that this was a population-level study. It did not directly connect individual COVID-19 infections with individual PE cases or determine patients' vaccination s
tatus.
Conclusions
The findings suggest that COVID-19 waves may create a delayed vascular burden extending beyond immediate respiratory illness, with pulmonary embolism risk becoming most apparent several weeks later. Better infection prevention could therefore potentially reduce serious downstream clotting complications.
The study findings were published in the peer reviewed journal: Public Health.
https://www.sciencedirect.com/science/article/pii/S003335062600332X
Read Also:
https://www.thailandmedical.news/articles/coronavirus
https://www.thailandmedical.news/articles/long-covid 
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