The Food and Drug Administration will likely require only a “matter of days” to look into reports of six patients who received the Johnson & Johnson vaccine experiencing blood clots and a “stroke-like illness,” officials said Tuesday.

“This timeframe will depend obviously on what we learn in the next few days, however, we expect it to be a matter of days for this cause,” Acting FDA Commissioner Janet Woodcock told reporters at a press briefing.

The FDA and the Centers for Disease Control and Prevention called for use of the shot to be paused Tuesday as they investigate rare blood clots that occurred among female patients after the vaccination.

“There have been six reports of a severe stroke-like illness linked to low platelet counts,” CDC principal deputy director Dr. Anne Schuchat told reporters.

One woman died and a second was hospitalized in critical condition.

The women, whose ages range from 18 to 48, had experienced low platelet counts along with the clots in veins that draw blood from the brain, officials said.

The cases occurred an average of nine days after they received the one-dose vaccine, the agency said.

Schuchat, who is the CDC Principal Deputy Director, said not all — if any — of the women had predisposing conditions.

“The review of six [cases] is difficult to make generalizations from. We’re going to have our expert committee take a careful look,” Schuchat told reporters.

Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, stressed that the cases were “extremely rare.”

The blood clots occurred out of the 6.8 million doses administered since shipments began in March.

Regulators said they wanted to alert patients and health care professionals about the condition known as cerebral venous sinus thrombosis since — when treated with the blood thinner heparin — it could be potentially “dangerous.”

“Treatment of this specific type of blood clot is different from typical treatments for other types of blood clots,” Marks told reporters.

“With cerebral venous sinus thrombosis, Heparin may be dangerous and alternative treatments need to be given, preferably under the guidance of physicians experienced in the treatment of blood clots seen in the United States.”

Officials encouraged both patients and healthcare professionals to be on the lookout for any warning signs

“If you’ve received a vaccine and develop severe headaches, abdominal pain, leg pain or shortness of breath, you should contact your health care providers and seek medical treatment,” Schuchat said.

“Now these cases are different from the mild flu-like symptoms, fever and so forth that many people experienced in the couple days after receiving the vaccine.”

The drugmaker said in a statement Tuesday that the “safety and well-being of the people who use our products is our number one priority.”

“We have been working closely with medical experts and health authorities, and we strongly support the open communication of this information to healthcare professionals and the public,” the company said.

It’s unclear whether Johnson & Johnson will stop any of its vaccine trials currently underway in the US.

In New York, people scheduled to have the Johnson & Johnson shot will either get the Pfizer vaccine or have their appointment rescheduled, officials said.

Federal officials said they don’t anticipate the setback to impact President Biden’s goal to vaccinate 200 million Americans in the first 100 days of his administration.

“I think the vaccine supply has become more abundant over time and I think this temporary pause is hopefully not going to have a large adverse effect on making those goals in a timely manner if at all,” Marks said.

Jeffrey Zients, the White House coronavirus coordinator, noted that the J&J shots only account for “less than 5 percent of the recorded shots in arms in the United States to date.”

The CDC’s Advisory Committee on Immunization Practices will convene Wednesday to discuss the cases and the FDA launched a probe.

Federal officials said the review will consider if there are certain “subsets of the population” that are at higher risk for the potential adverse reaction.

“We are committed to safety and transparency, and to expeditiously learning as much as we can so that further steps can be taken,” Schuchat said.

The pause comes after regulators in other countries raised concerns about a similar issue with another vaccine, developed by AstraZeneca and Oxford University researchers, which is not in use in the US.

“That pattern is very, very similar to what was seen in Europe with another vaccine, so I think we have to take the time to make sure we understand this complication and we address it properly,” Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, told the press conference.