Health officials have recommended a “pause” in the single-dose Johnson & Johnson COVID-19 vaccine to investigate reports of rare but potentially dangerous blood clots.

The Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) announced Tuesday that they were investigating unusual clots that occurred in patients anywhere from six to 13 days after vaccination. The FDA commissioner said the pause is expected to last a matter of days.

The clots reportedly occurred in veins that drain blood from the brain and occurred together with low platelets, the fragments in blood that normally form clots. All six cases reported were in women between the ages of 18 and 48. One person died, and all of the cases remain under investigation.

Over 6.8 million doses of the J&J vaccine have been given in the U.S., with the vast majority of patients reporting no or mild side effects.

According to the FDA, the cases under investigation appear similar to unusual clots that may be linked to the AstraZeneca vaccine, which has not yet been cleared for use in the United States. European regulators have say that the AstraZeneca risk appears to be lower than the possibility of developing clots from birth control pills.

Mass vaccination sites that are run federally will pause the use of the J&J shot, and states and other providers are expected to follow suit. The other two authorized vaccines, from Moderna and Pfizer, make up the vast share of COVID-19 shots administered in the U.S. and are not affected by the pause.

A CDC committee was planning to Wednesday to discuss the cases, and the FDA has also launched an investigation into the cause of the clots and low platelet counts.

FDA officials said that Tuesday’s action was not a mandate and doctors and patients could still use J&J’s vaccine if they decide to.

J&J said in a statement that it was aware of the reports of blood clots, but that no link to its vaccine had been established. The company also said it would delay the rollout of its vaccine in Europe as a precaution.