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ASH President: Hematologic Complications of COVID-19 Vaccines Are Rare

Blood clot experts encourage vaccination amid reports of extremely rare risk of vaccine-associated hematologic complications

(WASHINGTON, April 23, 2021) — Today, an advisory committee on vaccines convened by the Centers for Disease Control and Prevention recommended that the U.S. Food and Drug Administration lift the temporary pause on the Johnson & Johnson COVID-19 vaccine in the United States that was put in place in response to reports of extremely rare but serious blood clots. A warning label will be added to be reviewed at the time of administration.

In response to this recommendation, American Society of Hematology (ASH) President Martin S. Tallman, MD, of Memorial Sloan Kettering Cancer Center issued the following statement:

“Highly effective vaccines are the most powerful tool in our arsenal to end the COVID-19 pandemic. Based on existing evidence, the benefits of vaccination against COVID-19 significantly outweigh the extremely low risk of blood clots or other hematologic complications reported after vaccination. In fact, COVID-19 infection carries a much higher risk of blood clots —147,000 in 1 million hospitalized COVID-19 patients experience clots, compared to roughly two in 1 million individuals who received the Johnson & Johnson vaccine. Getting vaccinated against COVID-19 is the greatest protective measure one can take to avoid COVID-19-associated blood clots.

In very few instances, individuals who received the Johnson & Johnson or AstraZeneca vaccines developed rare but serious blood clots termed thrombosis with thrombocytopenia syndrome (TTS); formerly identified as vaccine-induced immune thrombotic thrombocytopenia or VITT. No evidence for a relationship between TTS and either mRNA vaccine (Moderna or Pfizer) has been uncovered. Upon first learning of this complication in early April, ASH mobilized leading clotting experts to help clinicians working on the front lines recognize and treat this rare condition. Consensus opinion on how to distinguish TTS from more common vaccine side effects is available through the ASH website, as well as recommendations on the best treatment approach —building on rapid laboratory and clinical research on a similar thrombosis syndrome —should a vaccinated individual develop this extremely rare condition.

ASH will continue to review new evidence on the intersection of hematology and COVID-19 and update guidance on COVID-19 treatment for front-line health care providers in real time.”


The American Society of Hematology (ASH) (www.hematology.org) is the world’s largest professional society of hematologists dedicated to furthering the understanding, diagnosis, treatment, and prevention of disorders affecting the blood. For more than 60 years, the Society has led the development of hematology as a discipline by promoting research, patient care, education, training, and advocacy in hematology. ASH publishes Blood (www.bloodjournal.org), the most cited peer-reviewed publication in the field, and Blood Advances (www.bloodadvances.org), an online, peer-reviewed open-access journal.

Amanda Szabo, American Society of Hematology
[email protected], 202-552-4914