The CDC and FDA jointly issued a recommendation on Tuesday for states to stop using Johnson & Johnson’s COVID-19 vaccine while they conduct an investigation into 6 cases of rare but serious blood clots that developed within two weeks of receiving the shot. The pause is also intended to give better guidance to health providers on potential symptoms to watch out for in those already given the vaccine.

The announcement isn't expected to have a significant impact on Massachusetts’ supply. State officials had already expected a significant drop in the J&J supply due to recent problems at the company's Baltimore production facility. To date, more than 181,000 people in Massachusetts have received the Johnson & Johnson vaccine, which accounts for just over 10 percent of those vaccinated in the state.

More than six million doses of the vaccine have been given in the United States alone, and experts say the pause — even for a rare number of cases — shows that the safety system for vaccines is working.

“This is how the surveillance system is supposed to work. Rare adverse events detected and fully investigated, and a decision made,” said Dr. William Moss, executive director of the International Vaccine Access Center at Johns Hopkins Bloomberg School of Public Health.

Yet, the call by federal health agencies for an immediate pause in the use of Johnson & Johnson’s single-dose COVID vaccine over cases of a rare blood clot problem, has raised many questions. Here’s what we know now:

Why did the FDA and CDC issue this pause?
Right now, these adverse events appear to be “extremely rare,” according to the two health agencies. Cerebral venous sinus thrombosis, or CVST, is a type of blood clot that forms in the brain. The common way to treat “thrombosis” or blood clots is with heparin, commonly known as a blood thinner, but officials on Tuesday advised against using it and suggested seeking alternative treatments. Dr. Anthony Fauci told reporters using heparin “would be a mistake in this situation because it could be dangerous and make the situation much worse.” The agencies want the health care community to “plan for proper recognition and management.”

Are they stopping the vaccine completely?
The pause is likely temporary and the acting FDA commissioner Janet Woodcock expects it will likely be “days” before the pause is lifted. The CDC and FDA have recommended this step “out of an abundance of caution” until they have completed investigations into the blood clot cases. It is a recommendation, and not a revoking of the current approval of the Johnson & Johnson vaccine, however, it is expected that states will pause use of it. It is possible that the vaccine could continue to be used after investigations are complete, but with different guidance on who should be given the vaccine. On Tuesday, Johnson & Johnson also said they would “proactively delay” the vaccine’s rollout in Europe.

Are there similar concerns about the Moderna and Pfizer vaccines?
There have been more than 180 million doses of Moderna and Pfizer vaccines administered and no reports of this particular clotting problem, according to FDA and CDC officials. However, this blood clotting issue is similar to the cases found with the Astra Zeneca vaccine, which is made using the same adenovirus technology, which relies on a type of cold virus to carry the spike gene of the virus into the body. AstraZeneca is not yet approved for use in the United States.

What are the symptoms of this potential blood clot? What should I do if I’ve had the vaccine and am worried about the problem?
Call your doctor with any concerns. If you have had the Johnson & Johnson vaccine, the period to watch is 1-3 weeks after you are vaccinated. Symptoms could be headaches, leg pain, abdominal pain and shortness of breath. However, experts note, flu-like symptoms which are common after all vaccines, typically clear within a week. Dr. Anne Schuchat, principal deputy director of the CDC, said while they understood the step being taken may be “very concerning to Americans who have already received the vaccine … the risk to them is very low at this time.”

What do we know about the six blood clot cases?
According to the FDA and CDC, these cases are a type of blood clot called cerebral venous sinus thrombosis (CVST), and was seen in combination with low levels of blood platelets. All six cases occurred among women between the ages of 18 and 48, and symptoms occurred 6 to 13 days after vaccination. On Tuesday, Dr. Peter Marks, director of the FDA Center for Biologics Evaluation and Research, called the problem “very similar to what was seen in Europe” with the AstraZeneca vaccine.

What will the impact be on supply and appointments made in Massachusetts? And on the overall U.S. supply?
Both CDC and White House officials said they’re currently working now with state and federal partners to get anyone scheduled for a Johnson & Johnson vaccine rescheduled for a Pfizer or Moderna vaccine. On Tuesday, in Massachusetts, the Lower Merrimack Valley Regional Collaborative — which covers 9 municipalities — announced it had cancelled a Thursday clinic with the Johnson & Johnson vaccine but those appointments could be rescheduled for the weekend with the Pfizer vaccine anytime after.

Jeffrey Zients, the White House COVID-19 response coordinator, said the announcement “will not have a significant impact on our vaccination plan,” and said with supply from Pfizer and Moderna, the U.S was still on track to continue its “current pace” of 3 million shots per day.