The Food and Drug Administration on Tuesday called on states to temporarily stop using Johnson & Johnson’s Covid-19 vaccine “out of caution” after six women in the US developed a rare blood clotting disorder in which one woman was dead and one the other critical was a condition.

“Right now, these adverse events seem extremely rare,” the FDA said in a joint statement with the Centers for Disease Control and Prevention. “The safety of COVID-19 vaccines is a top priority for the federal government and we take all reports of health problems following COVID-19 vaccination very seriously.”

All six cases occurred in women aged 18 to 48 years, with symptoms developing six to 13 days after receiving the shot. Doctors usually treat these types of blood clots with heparin, but health officials determined that it could be dangerous in this case and recommended a different treatment.

J&J said in a statement that “no clear causal link” was found between the blood clots and the vaccine, adding that it is working closely with regulators to evaluate the data.

People who received the J&J vaccine more than a month ago are at very little risk of developing blood clots, said Dr. Anne Schuchat, the CDC’s deputy chief executive officer, told reporters on a conference call Tuesday.

“People who have recently received the vaccine in the past few weeks should be aware that they are looking for symptoms,” she said. “If you have received the vaccine and have severe headache, stomach pain, leg pain, or shortness of breath, you should contact your doctor and see a doctor.”

Blood clotting generally occurs about a week after vaccination, but no more than three weeks afterwards, with a median time frame of 9 days, said Dr. Peter Marks, director of the FDA Center for Evaluation and Research of Biologics, on the call.

“We know these vaccines have flu-like symptoms for the first few days after vaccination, which can include headaches,” he said, adding that these are likely to be common adverse events. “If someone were presented to an emergency department with a severe headache or blood clot, it would be more likely that a vaccination history would be triggered,” and a care plan would be established. If the patient also has low platelets, doctors should consider that it may be caused by the vaccine, he said.

Johnson & Johnson COVID-19 vial and box at a vaccination site. Despite a small number of people who have experienced side effects, including blood clots, doses of the Johnson & Johnson vaccine are being given across the state of Florida.

Paul Hennessy | LightRocket | Getty Images

J & J’s shares fell nearly 3% in intraday trading.

The CDC will hold a meeting of the Advisory Committee on Vaccination Practices on Wednesday to further study the cases, federal health officials said on Tuesday. The FDA is also investigating the cases.

Last week, the European Medicines Agency said it had identified a possible link between the coronavirus vaccine developed by AstraZeneca and Oxford University and rare blood clotting problems. AstraZeneca has not received approval for use in the United States

Emer Cooke, executive director of the European Medicines Agency, said in a televised news conference last week that unusual blood clotting with low platelets would be added to the AstraZeneca vaccine product information as a “very rare” side effect, along with a number of other possible ones Side effects.

Isaac Bogoch, an infectious disease specialist who served on various drug data and safety monitoring panels, said J&J and AstraZeneca use the same platform for their vaccines. The J&J and AstraZeneca vaccines use an adenovirus, a common type of virus that typically causes mild cold symptoms.

“Obviously we don’t have all the information and we need to learn more about the mechanism and risk factors,” he told CNBC. “But of course we can’t sweep potential adverse events under the rug and it needs to be investigated thoroughly.”

Marks told reporters that researchers had not yet found a specific cause of blood clotting in J&J vaccine recipients, but they believed it was similar to other adenovirus vector vaccines.

“This is an immune response that occurs very, very rarely after some people get the vaccine, and that immune response activates the platelets and the extremely rare blood clots,” said Marks.

J&J submitted its Covid vaccine data to the FDA in February and, according to the agency, no specific concerns were identified at the time of analysis by age, race and comorbidities. The FDA said at the time that the most common side effects were headache and fatigue, followed by muscle pain, nausea, and fever.

It is unclear how the hiatus will affect J & J’s goal of shipping 100 million cans to the US by the end of May. The company was already plagued by manufacturing problems after a facility operated by Emergent BioSolutions ruined 15 million doses of the vaccine.

Dr. Kavita Patel, a former federal health agency, told CNBC that she believes the FDA’s recommendation is likely to have lasting effects on the nation’s efforts to fight the pandemic.

“This is a devastating blow to these J&J vaccination efforts in the US,” said Patel, a family doctor in Washington, in an interview on CNBC’s “Squawk Box”. Patel said the supply of Pfizer and Moderna’s two-shot vaccines will not be able to quickly meet the demand caused by the J&J hiatus. This will delay US vaccination efforts, she added.

Jeffrey Zients, President Joe Biden’s Covid Tsar, said the hiatus would not “significantly impact” the government’s goal of administering 200 million Covid shots within Biden’s first 100 days in office. He said J & J’s vaccine accounts for less than 5% of the gun shots recorded in the US to date.

“Based on the measures taken by the president earlier this year, the United States has secured enough doses of Pfizer and Moderna for 300 million Americans,” he said in a statement. “We have provided more than 25 million doses of Pfizer and Moderna every week for the past few weeks, and in fact we will be delivering 28 million doses of these vaccines this week.”