The following article features coverage from the American Society of Hematology 2021 meeting. Click here to read more of Hematology Advisor‘s conference coverage.

Vaccination against COVID-19 does not appear to increase the risk of deep vein thrombosis (DVT), according to research presented at the 2021 American Society of Hematology (ASH) Annual Meeting.

Previous research has shown that vaccinations against SARS-CoV-2, the virus that causes COVID-19, which are approved in the US are both generally safe and effective. While rare thrombotic events have been reported after the administration of some vaccines, no previous research has explicitly evaluated the causal relationship between vaccination and DVT risk. For this study, researchers aimed to compare the rates of DVT prior to and after vaccination against COVID-19.

Researchers evaluated clinical data from patients at least 18 years of age vaccinated against COVID-19 between November 2020 and June 2021. All patients had received at least 1 dose of a vaccine approved in the US.

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Overall, data from 382,527 patients were included. In this overall study population, the median patient age was 61 years, 55.3% of patients were male sex, and 64.2% had received the Pfizer vaccine, while 31.6% had received the Moderna vaccine. Although 4.3% (16,272 patients) received the Janssen vaccine overall, non-White individuals were more likely to receive Janssen (21%).

In the overall population, 5960 patients underwent 7265 upper and lower DVT venous duplex ultrasounds in the 90 days before and after vaccination. Acute DVT was noted by a natural language processing algorithm in 714 patients (11.1% of ultrasounds overall). This corresponded to an overall acute DVT rate of 1.86 per 1000 people, which the researchers noted accorded with the background epidemiologic rate.

Analysis suggested that there was no correlation between the probability of DVT and whether ultrasound took place prior to or after COVID-19 vaccination. Acute DVT occurred in 10.8% of ultrasounds after vaccination compared with 11.6% before vaccination (P =.28).

Although no differences were noted for the Moderna and Pfizer vaccines, patients who underwent ultrasound who received the Janssen vaccine were more likely to have a DVT before vaccination (18%) rather than after (7%; P =.0003). The presenter suggested that this finding may be due to over-testing among patients who received Janssen.

Disclosure: The study author(s) declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of authors’ disclosures.

Read more of Hematology Advisor‘s coverage of the ASH 2021 meeting by visiting the conference page.


Houghton DE, Padmanabhan A, Ashrani AA, et al. Deep vein thrombosis after COVID-19 vaccinations. Presented at ASH 2021; December 11-14, 2021. Abstract 291.