“Everyone knew it could cause a hit to vaccine confidence,” he said. Though the cases “could just be a statistical aberration,” the health authorities sought to exercise an abundance of caution.
Safety issues have emerged with other Covid vaccines that clinicians have learned to cope with, he said. For example, after a number of cases of anaphylaxis, a severe allergic reaction, were linked to mRNA vaccines, health authorities educated doctors on treatment.
Researchers think that the blood clots may be caused by a rare autoimmune reaction against the vaccine that leads to an unusual low platelets and severe clots. Just as heparin can in rare cases sensitize the immune system against platelets, the vaccines may create a similar reaction.
The vaccine-associated blood clots are “super rare” and show up with a very unusual patterns of clotting in the head or abdomen, said Mark Crowther, a hematologist and chair of the department of medicine at McMaster.
Unlike a stroke, where arteries bringing blood into the brain are blocked, with the vaccine-associated clots, the veins draining blood from the head get blocked, said Crowther, who’s also an officer of the American Society of Hematology. That’s one reason why patients often report severe headaches, he said.
In two studies published April 9 in the New England Journal of Medicine, a research team in Norway and another group in Germany and Austria found that patients who had severe clotting reactions to the AstraZeneca vaccine had antibodies to an important clotting protein called platelet factor 4.
Six of the 11 blood-clotting cases reported by the German and Austrian researchers were fatal, while three of the five people with blood clots studied by the Norwegian scientists died, according to the New England Journal of Medicine studies.
The blood-clotting disorder “is a new phenomenon with devastating effects for otherwise healthy young adults,” the Norwegian researchers from Oslo University Hospital concluded.
Hints of a connection to clotting may have arisen during J&J’s final tests. In a report to the FDA, J&J said that a 25-year-old male trial volunteer in its phase 3 trial had suffered blood clotting in the brain, resulting in cerebral hemorrhage. After concluding that the event was unrelated to the vaccine, J&J resumed its U.S. trial.
Overall, there were 14 cases of clotting and bleeding events patients who received J&J’s vaccine in the trial, according to a report to the FDA. There were 10 in the group that received the placebo. Events so rare that they occur once in only thousands to millions of patients are very difficult to tie to a specific cause, said Weitz, the McMaster professor.
—With assistance from Stephanie Baker, Riley Griffin and Natalie Lung.
This article was provided by Bloomberg News.