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VERIFY: Did Johnson & Johnson have a normal trial size?

With emerging concerns over whether the J&J vaccine is linked to rare, but serious blood clots, some have questioned whether the trials tested enough people.

CHARLOTTE, N.C. — While federal officials try to determine how best to move forward from the pause on Johnson and Johnson's COVID-19 vaccine, claims continue to swirl about the vaccine and whether these developments prove the process was rushed or the company failed to test the product on enough people.

Tuesday, the Centers for Disease Control and Prevention and the Food and Drug Administration issued a joint statement calling for a pause on giving the vaccine, following revelations of rare, but serious blood clots, coupled with low platelet counts in a handful of vaccine recipients.

It is still unknown whether the vaccine is the cause of the clotting condition, and right now, its incidence rate is 0.0009%, since there have been six of these cases out of roughly 7 million J&J vaccines administered in the U.S.


Did Johnson and Johnson have a normal phase 3 clinical trial size?


Yes, the Johnson and Johnson phase 3 trial had approximately 40,000 participants, which is similar to the Pfizer and Moderna trials and similar to other vaccines.

According to the CDC, a typical phase 3 trial has anywhere from hundreds to thousands of volunteers.

Credit: CDC

"(The trials) were the same size as we would typically do, even if a vaccine wasn't on a not-accelerated path," Dr. Tony Moody, a professor of pediatrics with the Duke Human Vaccine Institute, said.

So even having a normal-sized trial, would one expect such a rare event be picked up in testing, with enough conclusive evidence?

Based on the apparent incidence of the clotting condition, Dr. Brannon Traxler, Interim Public Health Director with South Carolina's Department of Health and Environmental Control, thinks it is very possible for such a condition to go undetected.

"A side effect that's one-in-a-million, might not emerge until millions of people have received that vaccine," Traxler said.

In fact, FDA briefing documents on the Johnson and Johnson trial show there was one case of the specific clot in question, cerebral venous sinus thrombosis, during the clinical trials, but the evidence was not enough to directly link it to the vaccine. 

That clot happened to a 25-year-old man, for whom the study was paused. The brief states the man received several tests, but "no clear cause was identified" and the man had other risk factors that could have caused the condition. The investigator's final assessment was that the situation was unrelated to the vaccine.

During the Wednesday meeting of the CDC's Advisory Committee on Immunization Practices, Johnson and Johnson also revealed to the panel that a second CVST happened during the trials, but it was in a 24-year-old woman who was on the placebo.

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