Federal health officials predict the rollout of Johnson & Johnson’s coronavirus vaccine may be resumed within “days” but restrictions on certain age groups may be considered.

The Food and Drug Administration called for a pause on the vaccine’s rollout on Tuesday after six blood-clot cases were reported among the 7 million people who have been given the shot.

That makes the reported complications beyond infrequent: less than one in a million. Nevertheless, the federal government will immediately stop using the vaccine at all federally-run vaccination sites across the nation. State health officials have been advised to follow suit and not use the J&J shots they have in storage. New York was the first state to confirm it was immediately pausing its rollout but said all appointments scheduled for Tuesday can go ahead with the Pfizer shot.

In a Tuesday statement, the FDA said it made the decision after six reported cases of what it called “rare and severe type of blood clot in individuals after receiving the vaccine,” but it stressed: “Right now, these adverse events appear to be extremely rare.”

In a joint media call on Tuesday between the FDA and the Centers for Disease Control and Prevention, acting FDA commissioner Dr. Janet Woodcock said she expected the pause to last “a few days.” The aim, officials said, is largely to give time for regulators to reach out to clinicians so they know how to diagnose and report similar cases.

Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, emphasized that the pause is a recommendation, not a mandate. “If an individual healthcare provider has a conversation with an individual patient... we aren’t going to stop that provider from administering the vaccine,” he said.

However, in a White House press conference later on Tuesday, U.S. National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci was unwilling to ratify Woodcock’s prediction, stating that it was “too premature” to even say whether the J&J vaccine would be allowed to be used after the pause.

Some seven million Americans have received J&J’s vaccine to date, and around nine million more have been shipped out, according to data from the CDC. The shots make up a key part of the Biden administration’s vaccine rollout because they only require a single dose and are easier to store than others.

However, health officials said that the pause is unlikely to have a significant impact on U.S. vaccine distribution. CDC Principal Deputy Director Dr. Anne Schuchat noted that J&J has been supplying a “great minority” of vaccine doses.

Health officials told The New York Times that the six U.S. recipients developed a rare disorder involving blood clots within about two weeks of their shots. All of them were women between the ages of 18 and 48. One is reported to have died and another remains in serious condition in a hospital.

The FDA and CDC are now expected to examine any possible link between the vaccine and the disorder, and decide whether—and how—to continue the J&J rollout.

Fauci said that the FDA and CDC are looking into potentially excluding certain age groups. “It is entirely conceivable... that there may be some restrictions in age group,” he said. This mirrors the approach taken by a number of European countries with the AstraZeneca vaccine. Some European countries, like France and Italy, have imposed or are considering imposing age limits on who can receive it.

Schuchat acknowledged that the pause could be concerning news for those that have already received the J&J vaccine. “People who got the vaccine within the last couple of weeks” should monitor for symptoms like “severe headaches, abdominal pain, leg pain, and shortness of breath,” she said. Individuals experiencing these symptoms should contact their medical provider. However, those that received the vaccine a month ago were at very low risk, Schuchat added.

Johnson & Johnson said in a statement that there’s no evidence to confirm its vaccine caused the very rare blood-clot cases. “We are aware that thromboembolic events including those with thrombocytopenia have been reported with COVID-19 vaccines,” the company reportedly said. “At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine.”

The announcement follows a similar setback in Europe, where the AstraZeneca vaccine was paused to investigate its possible link to blood-clot events. After studying 86 European cases, the European Medicines Agency decided that the benefits of the vaccine outweighed the risk, even with the possibility that blood clots are an extremely rare side-effect.

Marks said on Tuesday that the potential J&J side effects closely resemble similar blood-clotting incidents reportedly associated with the AstraZeneca vaccine, calling the parallel “plainly obvious.” However, health officials stressed that these events are extremely rare and that the pause was being recommended out of an “abundance of caution.”

When asked during Tuesday’s press conference if the pause would worsen U.S. vaccine hesitancy, White House Coronavirus Response Coordinator Jeff Zients acknowledged the issue.

“Hesitancy is a challenge,” he said. The solution, he suggested, could be in “community-level” outreach. “People trust their local doctor, their faith leader.”