Scientists from Johnson & Johnson said on Friday that there is "insufficient" evidence of a "casual relationship" between the company's COVID-19 vaccine and the rare cases of blood clots that have prompted a pause in the vaccine's distribution.
In a letter to the editor of The New England Journal of Medicine, researchers with the company said that "at this time, evidence is insufficient to establish a causal relationship between these events" and the vaccine.
The U.S. Centers for Disease Control and Prevention had identified six cases of blood clots out of over 7 million inoculations using the Johnson & Johnson vaccine throughout the United States, and a pause had been placed on its distribution "out of an abundance of caution."
The blood clots under investigation are highly unusual. Known as cerebral venous sinus thrombosis (CVST), they occur in strange places, in veins that drain blood from the brain, and in people with abnormally low levels of clot-forming platelets.
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"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."
The six cases raised an alarm bell because that number is at least three times more than experts would have expected to see even of more typical brain-drainage clots, said CDC's Dr. Tom Shimabukuro.
"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.
The CDC’s Advisory Committee on Immunization Practices intends to meet on April 23 to provide guidance on Johnson & Johnson’s vaccine.