Blood Clots Occur With Pfizer and Moderna Vaccine at Similar Rate to AstraZeneca’s: Study A rare type of blood clot occurs at a similar post-vaccination rate among those who get Pfizer or Moderna shots and people who receive the AstraZeneca COVID-19 vaccine, according to a newly published study. Researchers found that four in a million patients experienced cerebral venous thrombosis (CVT) after getting a shot produced by Pfizer or Moderna, compared to the five in a million in those who received the AstraZeneca vaccine. The University of Oxford also has a five in million rate. The study, conducted by scientists from the university, was observational, using electronic health records to explore the incidence of the rare type of clot in people two weeks following COVID-19 diagnosis, or influenza, or receipt of vaccines from Pfizer or Moderna.