Johnson & Johnson: 'Insufficient' evidence of 'causal relationship' between COVID-19 vaccine, blood clots

Johnson & Johnson scientists said on Friday that there’s currently “insufficient” evidence of a “causal relationship” between the company’s COVID-19 vaccine and the rare cases of blood clots that prompted a pause in distribution. 

Company researchers wrote a letter to the editor of The New England Journal of Medicine published Friday that said "at this time, evidence is insufficient to establish a causal relationship between these events” and the Johnson & Johnson vaccine. 

The vaccine manufacturer’s comments follow the Tuesday decision by the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) to call for a pause in the rollout of Johnson & Johnson’s vaccine amid six blood clot cases. Every state has since suspended Johnson & Johnson vaccinations, and the company has halted clinical trials.


The six documented cases, out of the more than 6.8 million that received the Johnson & Johnson vaccine in the U.S., involved cerebral venous sinus thrombosis (CVST) and low blood-platelet counts, leading to one death. 

“CVST is a very rare health condition, and thus far, events reported in recipients of the ... vaccine are occurring within the range of published background incidence,” the researchers wrote. “It is important to note that the incidence of CVST associated with low platelets is unknown and is considered by the FDA and the CDC to be extremely low."

“We continue to work closely with experts and regulators to assess the data, and we support the open communication of this information to health care professionals and the public,” the researchers concluded. 

Johnson & Johnson noted that out of more than 75,000 participants in the clinical trial program for its vaccine, one case of CVST emerged, prompting the manufacturer to pause the program to review the case. When “no clear causality was established,” Johnson & Johnson continued with the trial with permission from the data and safety monitoring board. 

Following this week’s pause, an independent advisory panel at the CDC delayed voting to make a recommendation on the vaccine in question in order to gather more evidence on the rare cases. 

The CDC’s Advisory Committee on Immunization Practices intends to meet on April 23 to provide guidance on Johnson & Johnson’s vaccine. 


Some cases involving recipients of the AstraZeneca vaccine, which is approved for use in Europe, developing rare blood clots have also emerged in recent weeks. 

Both the AstraZeneca and Johnson & Johnson vaccines are adenovirus-based, but Johnson & Johnson scientists said the vectors and protein inserts used in each shot are “substantially different.”

Concerns have grown over whether the delay for the Johnson & Johnson vaccine will impact vaccine confidence and hesitancy across the country.