“Right now, we believe these events to be extremely rare, but we are also not yet certain we have heard about all possible cases, as this syndrome may not be easily recognized as one associated with the vaccine,” Dr. Rochelle P. Walensky, the C.D.C. director, said at a White House news conference on the pandemic on Wednesday.
During the panel discussion, experts noted that the “risk window” for the condition among vaccine recipients was still open and that new cases might emerge, because nearly 3.8 million people had received the shot within the last two weeks. In the six women, the severe clotting developed within about two weeks of the shot.
Other experts encouraged dissemination of health information on diagnosis and treatment of the condition so that awareness would spread among doctors, emergency rooms and people who had received the vaccine. A key point is that the blood-thinner heparin, a common treatment for clots, can harm these patients and should not be used.
Officials also noted that people with the condition needed to be treated as soon as possible because the clots were so serious. Some patients needed invasive procedures to remove large clots from blood vessels in their brains.
Several panel members reiterated that two other vaccines — from Moderna and Pfizer-BioNTech — are available, neither associated with the clotting problem, so continuing the pause would not stop most people in the United States from being vaccinated.
At the news conference, Jeffrey D. Zients, the White House’s pandemic coordinator, said that the pause would not generally interrupt the momentum of the country’s vaccination campaign.
“In the very short term, we do expect some impact on daily averages as sites and appointments transition from Johnson & Johnson to Moderna and Pfizer vaccines,” he said. “We have more than enough Pfizer and Moderna vaccine supply to continue or even accelerate the current pace of vaccinations.”