evidence: The Link Between J&J’s COVID Vaccine and Blood Clots: What You Need to Know [Originally published: April 21, 2021. Updated: May 17, 2023.] [...] The Pfizer-BioNTech and Moderna COVID-19 vaccines, both of which are mRNA vaccines and use a different method to protect against the virus, have not been linked to blood clots at this time. [...] COVID-19 infection seems to cause the blood vessels and the blood itself to behave in a way that promotes formation of blood clots, which is likely a key driver for poor outcomes, sometimes damaging vital organs and even leading to death, Dr. Chun says.