The cases, in women under age 50, were reported out of 7.2 million US doses of the J&J vaccine administered – a risk federal officials and immunology experts said was extremely low given the novel coronavirus' heavy toll. One of the six women died. So far, more than 562,362 people in the United States have died from the novel coronavirus.
J&J's single-dose shot is less widely used in the United States compared to the 185 million doses delivered of Moderna and Pfizer/BioNTech's two-shot vaccines. However, it has been seen as a critical option to expand protections to harder-to-reach populations.
The US Centers for Disease Control and Prevention's immunization panel will review the clotting cases and make recommendations when it convenes at 1:30 p.m. (1730 GMT). The Food and Drug Administration, which approved the vaccine, will then review the analysis and determine the next steps.
Biden administration officials have said they do not expect the pause of J&J's vaccine to impact its fight against the pandemic, citing enough doses of the two other vaccines to stay on track. The FDA said the halt should only last a few days and would help physicians understand and address any clot risk, which requires a different treatment than typical clots.
Dr. Anthony Fauci, President Joe Biden's chief medical officer, on Wednesday told CNN the pause would allow regulators to see if there were other possible clot events but that he did not think there would be many more such cases.
The FDA's pause "very well may be quite temporary," he said separately on MSNBC. Following the CDC review, the FDA could decide to resume use of J&J's vaccine, allow its use with some changes, or decide against its continued use, he said.
Fauci said it was up to the regulatory agency but that "very likely, they'll say we looked at it, and now we will go back, maybe make some modifications."
J&J has said no clear causal relationship has been established between the clots and its vaccine, but that it was working closely with regulators in the United States and Europe, where it also voluntarily paused its rollout.
Its shares were down less than 1% in premarket US trading on Wednesday, after closing down 1.3% on Tuesday.
Some health experts worry the pause may raise public doubts about US vaccination efforts, though Fauci and other top US health officials have said it should give people confidence that regulators take safety seriously.
"These pauses are actually common with new vaccines and drugs are rolled out. This is not unusual. We're just doing the due diligence to make sure everything is safe so we can continue with our vaccination efforts," US Surgeon General Vivek Murthy told CBS News' "CBS This Morning" program.