Health authorities in states across the country moved swiftly Tuesday to halt the use of Johnson & Johnson's coronavirus vaccine, following the advice of federal health agencies, which called for a pause while they review cases of rare blood clots in people receiving the shots.

U.S. officials said they are reviewing six reported cases of a "rare and severe type of blood clot" out of more than 6.8 million people in the country who have received the Johnson & Johnson vaccine.

New York, Washington D.C., Maryland, Virginia, New Jersey, Massachusetts, Connecticut, Ohio, Texas, Kansas, Missouri, Georgia and Indiana are among the states that said they would temporarily stop using the vaccine. 

But the responses differed for how they would deal with existing appointments. States had already been preparing for a significant decrease in the supply of Johnson & Johnson's vaccine

New York’s health commissioner Howard Zucker said the state would stop using the Johnson & Johnson vaccine, but advised people not to cancel their appointments. Instead, the state will be using supplies from either Pfizer or Moderna.

In D.C., health officials said people with appointments would be contacted to reschedule.

The decision to recommend a pause caught states by surprise, but officials from the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention said they acted quickly because of the need to alert health providers so they can treat patients who are affected.

"This was taken rapidly in order to honor our commitment to the American public to ensure that any safety signal that came up during this vaccine rollout was fully addressed in a transparent manner," Peter Marks, a top FDA official in charge of vaccines, said during a call with reporters.

"The time frame will depend obviously on what we learn in the next few days, however we expect it to be a matter of days for this pause," acting FDA Commissioner Janet Woodcock said.

Walgreens and CVS, two of the nation's largest retail pharmacy chains, also said they would stop using Johnson & Johnson's vaccine.  

In a statement, CVS said the company is canceling all appointments for people who were scheduled to receive a Johnson & Johnson COVID-19 vaccine, and "will follow up with affected customers to reschedule their appointments as soon as possible."

Walgreens said the company is "reaching out to patients with scheduled appointments and rescheduling vaccinations from other manufacturers, as supply allows."

The Department of Defense also said it was going to pause all inoculations with the Johnson & Johnson vaccine. 

"We are communicating this pause to our military health facilities and are reviewing our global vaccine distribution to address this issue and ensure we can continue to provide vaccines to our DoD population at home and abroad," Pentagon press secretary John Kirby said in a statement.