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.
The total incidence of CVT was 39 per million people.
While the identification of a similar risk from Moderna and Pfizer vaccines for clots is new, “we cannot conclude that the mRNA vaccines studied here are associated with an increased risk of CVT; far larger samples are needed to address this question,” researchers wrote.
The study has not yet been peer-reviewed.
A number of post-vaccination clots were identified in various countries since last month in people who got an AstraZeneca shot, prompting a review by the European Union’s drug regulator.
The European Medicines Agency in early April concluded that there was a “possible link” between the shot and the clots, advising AstraZeneca to mention the clots as a possible side effect.
Post-vaccination clots in patients who received Johnson & Johnson’s vaccine triggered a near-nationwide pause in using the shot in the United States this week. Those issues are similar to the ones seen in people who received an AstraZeneca shot, U.S. health officials have said.
But U.S. officials had also said neither Moderna’s or Pfizer’s shot were linked to CVT, at least in combination with low blood platelet levels, or thrombocytopenia.
“To be very clear, these types of reactions—blood clots in combination with low platelets—are not being seen with the authorized vaccines from Pfizer and Moderna,” Centers for Disease Control and Prevention Director Rochelle Walensky told a virtual briefing this week.
Moderna and Pfizer had also said that assessments of safety data showed no link with either CVT or thrombotic events. Neither company returned requests for comment on the new study.
The records for the new study came from TriNetX Analytics, a health records network that provides anonymized data from 59 healthcare organizations, primarily in the United States. The network has records for 81 million patients. Researchers looked at the records of 513,284 patients, some of whom were diagnosed with COVID-19 between Jan. 20, 2020, and March 25. Others received a flu diagnosis during the same period of time while a third cohort received a COVID-19 vaccine.
The risk of experiencing a CVT was eight to 10 times higher in the two weeks after being diagnosed with COVID-19, versus people who got the flu or people who received one of the two vaccines, both of which are built on a messenger RNA platform.
COVID-19 is the disease caused by the CCP (Chinese Communist Party) virus.
“We’ve reached two important conclusions. Firstly, COVID-19 markedly increases the risk of CVT, adding to the list of blood clotting problems this infection causes. Secondly, the COVID-19 risk is higher than see with the current vaccines, even for those under 30; something that should be taken into account when considering the balances between risks and benefits for vaccination,” Paul Harrison, professor of psychiatry and head of the Translational Neurobiology Group at the University of Oxford, said in a statement.