The U.S. Centers for Disease Control and Prevention and the U.S. Food and Drug Administration are calling for a pause on the use of Johnson & Johnson's single-shot coronavirus vaccine over concerns it may be linked to blood clots.

Officials said they have identified six people who developed a "rare and severe" type of blood clot after receiving the Johnson & Johnson vaccine. Over 6.8 million doses of the vaccine have already been administered in the U.S.

The six cases were in women between the ages of 18 and 48, and the symptoms started showing up as early as six days after they were inoculated.

The FDA and the CDC said they will examine the cases and have urged states to hold off on administering the vaccine until they have completed the review.

"CDC will convene a meeting of the Advisory Committee on Immunization Practices (ACIP) on Wednesday to further review these cases and assess their potential significance," the two health agencies said in a joint statement. "FDA will review that analysis as it also investigates these cases. Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution. This is important, in part, to ensure that the health care provider community is aware of the potential for these adverse events and can plan for proper recognition and management due to the unique treatment required with this type of blood clot."

The agencies said that symptoms of the blood clot include severe headache, abdominal pain, leg pain, and shortness of breath. They advised anybody who experiences those symptoms to contact their health care provider.

The pause should not impact the goal of administering 200 million vaccines within President Joe Biden's first 100 days in office.

"Based on actions taken by the President earlier this year, the United States has secured enough Pfizer and Moderna doses for 300 million Americans," White House Coronavirus Response Coordinator Jeffrey Zients said in a statement. "Over the last few weeks, we have made available more than 25 million doses of Pfizer and Moderna each week, and in fact this week, we will make available 28 million doses of these vaccines."

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