COVID-19 vaccines have been linked to a very rare blood clot condition called thrombosis with thrombocytopenia syndrome (TTS), primarily with the Johnson & Johnson (J&J) and AstraZeneca vaccines
. The risk of developing TTS is extremely low, and the risk of blood clots from a COVID-19 infection is significantly higher than from the vaccine.
Confirmed TTS cases for specific vaccines
Johnson & Johnson (Janssen)
- Total cases: As of March 2024, approximately 57 cases of TTS were reported in the U.S. among the roughly 14 million people who received the J&J vaccine. This is equivalent to about 4 cases per million doses.
- Fatalities: Nine deaths from TTS were reported in the U.S..
- Demographics: The highest risk was observed in women aged 18 to 49, with a rate of 8.7 cases per million doses. Following this data, the Centers for Disease Control and Prevention (CDC) began recommending mRNA vaccines over the J&J vaccine, which is no longer available in the U.S..
AstraZeneca (Covishield)
- Risk rate: In the UK, reports from June 2021 showed about 10 people developed TTS for every million doses of the AstraZeneca vaccine, a risk rate of 0.001%.
- Global perspective: By May 2024, the risk in younger adults was estimated to be around 1 in 100,000 doses, with an even lower risk for older adults. Concerns over rare blood clots led several countries to pause or restrict its use.
- Current status: The European Commission withdrew the vaccine's marketing authorization in March 2024.
Comparison with other COVID-19 vaccines
- mRNA vaccines (Pfizer and Moderna): The rare blood clot syndrome TTS has not been linked to the Pfizer or Moderna mRNA COVID-19 vaccines. Studies have found a slightly increased risk of blood clots after receiving these vaccines, but the risk is minor compared to the risk associated with a COVID-19 infection.
- Risk-benefit analysis: The risk of blood clots and other cardiac complications is significantly higher after a COVID-19 infection than after vaccination. For most individuals, the benefits of COVID-19 vaccination in preventing severe disease far outweigh the extremely low risks of vaccine-related blood clots.
Context for reported numbers
The number of blood clots potentially linked to vaccines varies by region and reporting system. For example, a 2021 study analyzing reports from Europe's EudraVigilance database found over 3,400 thrombotic adverse events following vaccination with AstraZeneca, Pfizer, and Moderna vaccines, but reporting systems like this cannot determine causality.
Therefore, total numbers can be misleading without understanding the extremely low incidence rate and acknowledging that most reported adverse events are not caused by the vaccine. |
