Dangerous Clots: Researchers Solve Mystery Following COVID Vaccination
Reports of rare, sometimes fatal, blood clots following certain COVID-19 vaccinations caused concern during the pandemic. Now, an international research team has clarified how these cases occurred – and why they only arose with specific vaccines.
The findings, published on February 17, 2026, in the New England Journal of Medicine, focus on a condition known as vaccine-induced thrombotic thrombocytopenia syndrome (TTS), also referred to internationally as VITT. This syndrome involves the simultaneous development of dangerous blood clots (thromboses) and a deficiency in platelets, which are normally responsible for blood clotting.
TTS was an extremely rare side effect. In the United States, approximately one in 200,000 people who received the Johnson & Johnson COVID-19 vaccine developed the syndrome. In the United Kingdom, around three in 100,000 individuals experienced it after vaccination with the AstraZeneca vaccine. Both of these vaccines are categorized as vector vaccines.
These vaccines utilized harmless adenoviruses – common cold viruses – as a delivery system to transport genetic material from the coronavirus into human cells. The immune system responded by producing antibodies against the coronavirus.
Understanding the mechanisms behind these rare but serious side effects is crucial for improving vaccine safety and public confidence in immunization programs. The research provides valuable insights that could inform future vaccine development and monitoring strategies.
A separate report highlights the continued importance of staying up-to-date with vaccinations, noting that a seasonal refresh of protection is beneficial regardless of prior vaccination or infection status. The highest level of protection against severe infection requiring hospitalization is observed in the months immediately following vaccination, according to Prim. Priv.-Doz. Dr. Arschang Valipour.
Experts also previously confirmed that approved vaccines remain effective against the Delta variant, as reported in July 2021.
