American health officials called for a “pause” in the use of the COVID-19 vaccine made by drug maker Johnson & Johnson, known as J&J. They said six people developed rare cases of blood system blockages, known as blood clots, after receiving the vaccine. The Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) released a joint statement Tuesday saying they are studying the cases. The agencies said six women, between the ages of 18 and 48, had blood clots from six to 13 days after vaccination. The FDA had approved the J&J vaccine for emergency use in late February. So far, nearly 7 million shots have been given out, most with few or no side effects. Health officials said anyone who has pain in their head, legs, in the middle part of their body or trouble breathing after receiving the J&J shot should contact their doctor. They also warned that a usual treatment for blood clots, the drug heparin, might be dangerous. They advised that healthcare workers should use different treatments for side effects from the vaccine. “I’d like to stress these events appear to be extremely rare. However, COVID-19 vaccine safety is a top priority for the federal government,” said FDA Acting Commissioner Janet Woodcock at a news conference on Tuesday. She expected the pause to last for a few days. The U.S. health agencies said their action was not an order to stop using the J&J vaccine. They said healthcare providers and patients could decide whether or not to receive the vaccine. However, several American states, including New York and Minnesota, said they would immediately stop using the vaccine until further study is completed.