Patient Education on Vaccines: Putting the COVID Shot's Blood Clot Risks into Perspective

With the many questions patients have about the rare but serious risk of blood clots after COVID-19 vaccination, patient education on vaccines is essential.

When the FDA briefly paused administration of the Johnson & Johnson (Janssen) COVID-19 vaccine in April 2021, it left many patients — particularly women — worried about the risks of a rare but serious blood clot called cerebral venous sinus thrombosis (CVST).


While the benefits of vaccination still far exceed the risks, the pause came at a time when vaccine hesitancy has been materializing across many regions. Many questions have arisen as the vaccine rollout continues, and patient education on vaccines remains essential.


What Happened With Johnson & Johnson?


Between 6 and 15 days post-vaccination, CVST had been seen in tandem with a condition called thrombocytopenia, which involves low levels of blood platelets, reports the Centers for Disease Control and Prevention (CDC). By mid-April, this clotting disorder had been reported in 6 patients out of 6.8 million doses of the single-dose vaccine. Nine more cases have been reported since to make a current total of 15 patients, according to the FDA. One person has died.


Of the original 6 cases, all were among women between ages 18 and 59 — raising questions about the unique effects of this vaccine formulation on women's health.


Potential Causes Under Consideration


Researchers have identified similarities between these clots and heparin-induced thrombocytopaenia (HIT), a rare side effect of the blood thinner heparin. However, because none of the vaccinated patients had taken heparin, scientists are still unclear about what specific mechanism could have triggered the immune response that causes the clotting disorder.


One viral immunologist told Nature that the issue could stem from any number of issues related to the specific vaccine type, which involves an inactivated adenovirus to carry genetic material to the cells.


Notably, the AstraZeneca COVID-19 vaccine — which is also adenovirus-based — has had similar clotting concerns. The Pfizer/BioNTech and Moderna vaccines, which rely on mRNA technology rather than adenovirus packaging, have not been linked to these effects.


Effects on Women's Health


Women who take hormonal birth control already face a higher risk of blood clots, but those types of clots — which tend to occur in the legs — are different from the cerebral clots seen among patients taking the Johnson & Johnson vaccine. Given these differences, the CDC has suggested that taking birth control does not necessarily increase a patient's risk for CVST after getting a Johnson & Johnson COVID-19 vaccine.


Still, female patients under 50 years old should be made aware of the risks of the Johnson & Johnson vaccine, even if they are exceptionally low. (The reported risks are lower than the clotting risks associated with hormonal contraceptives — although this comparison has been debated). Two-dose vaccines, including those from Pfizer/BioNTech and Moderna, have not been associated with these risks.


Making Informed Choices Through Vaccine Education


The rare risks of CVST join other sex-based considerations that have been noted amid vaccination for COVID-19, including potential temporary disruptions to the menstrual cycle as reported by research published in BMJ. However, there is no evidence that COVID-19 vaccines impact fertility.


The American College of Obstetricians and Gynecologists recommends that all women have access to COVID-19 vaccines, including anyone who is breastfeeding, pregnant or trying to become pregnant.


Ultimately, many patients are likely to have questions about their vaccination choice. Now more than ever, patient education on vaccines can help fill in the knowledge gaps so that everyone can make informed decisions.