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Federal officials on Friday lifted a 10-day “pause” placed on Johnson & Johnson’s Covid-19 vaccine, allowing its use to resume after a small number of unusual clotting events were reported among people who had received it.

The decision, announced by the Food and Drug Administration and the Centers for Disease Control and Prevention, came just two hours after an advisory committee to the CDC recommended the United States resume use of the vaccine without any restrictions on who should be offered it.

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“We are confident that this vaccine continues to meet our standards for safety, effectiveness, and quality,” acting FDA Commissioner Janet Woodcock said in a statement.

The U.S. pause was implemented after six reports emerged of people — all women — suffering rare and dangerous blood clots after being vaccinated with the J&J vaccine. All also had low platelets, a condition called thrombocytopenia. The unusual combination of the two, which has also been seen with AstraZeneca’s Covid-19 vaccine, is being called thrombosis with thrombocytopenia syndrome, or TTS.

CDC Director Rochelle Walensky said there were 1.9 cases of blood clotting per million people who had been vaccinated.

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The path recommended by the Advisory Committee on Immunization Practices and taken by the FDA and CDC — to return the vaccine to use without limitations on who can get it — diverges from the one chosen by a number of other countries regarding the AstraZeneca vaccine, which has also been tied to blood clots in rare instances. (That vaccine is not authorized for use in the U.S.) A number have recommended its use be limited to older adults; in the case of the U.K., the recommendations is people 18 to 29 years old be offered other vaccines when possible.

ACIP, as the panel is known, found that Johnson & Johnson’s vaccine is safe for the vast majority of people and is saving lives. Even for the age group that appears at highest risk — women aged 30 to 39 — the risk from Covid is greater than the risk of developing TTS after getting the J&J vaccine, CDC scientists told the committee.

Going forward, though, the vaccine’s label — and the factsheet given to people who receive it — will warn of the rare risk, urging people who develop worrying symptoms a number of days after vaccination to seek medical care. Those symptoms include headache, shortness of breath, bruising, and persistent abdominal pain.

“I think the decision was a very pragmatic one, and a very transparent one, a very clear one, and a correct one,” said Naor Bar-Zeev, a statistical epidemiologist at Johns Hopkins Bloomberg School of Public Health. “It’s a real risk and it’s a serious event if it occurs. But it’s very unlikely to occur.”

“Had we had an endless supply of any vaccine, then of course it would be ideal for younger women to receive vaccines that have not been found to be associated with clotting,” Bar-Zeev said. “But in the absence of that reality it is still the right decision for any given woman who is considering her own risk benefit-risk balance.”

The committee voted 10 to 4 to recommend use of the vaccine; one member who is involved in some Covid vaccine clinical trials abstained. The votes against the motion were not objections to resumption of use of the vaccine, but reflective of the feeling of some members of the committee that the recommendation should have included some information about the risk the vaccine may pose to some people — a risk group currently defined as predominantly women under the age of 50.

“This is an age group that is most at risk that is getting this vaccine predominantly to save other people’s lives … not their own,” said Sarah Long, a professor of pediatrics at Drexel University College of Medicine. “I’ve very sorry that we haven’t chosen to put up front the knowledge that we have: That this is unique, it’s clustered, it’s almost certainly related to the vaccine and there are options.”

Beth Bell, a professor of global health at the University of Washington and chair of the ACIP’s Covid-19 vaccine work group, had favored another option, which would have involved recommending resumption of use of the vaccine while also noting women under the age of 50 should be aware of the risk and may want to choose another vaccine. That option was not put to a vote and Bell voted to support the stripped-down recommendation.

“I am concerned that the consumers — and women in this age group in particular — will not be adequately informed just by the FDA [emergency use authorization] factsheets,” Bell said after the vote.

During the discussion, a number of members of the committee expressed worries that women who want to forgo the J&J vaccine might have a difficult time finding another option. Many clinics do not stock multiple brands of vaccine at once, they said, and a choice not to take the J&J will in some cases be a choice not to get vaccinated on that particular day. “Women are not going to be offered … across the country an option to a second vaccine and that’s going to place certain women at a disadvantage,” said ACIP chairman José Romero, who nonetheless voted for the recommendation to lift the pause. “And they’re going to have to shop for vaccine.”

Before Friday’s meeting, there had been six reported cases of the clotting problems, all of them among women between the ages of 18 and 48 who had been given the vaccine.

That number has now risen to 15, Tom Shimabukuro, a CDC vaccine safety expert, told the committee. The number may rise; Shimabukuro said “just under 10” possible additional cases are being investigated. Of the 15 cases so far, 12 involve a clot in the brain called a cerebral venous sinus thrombosis, or CVST; the other three people had a variety of other clots, also serious. Three of the confirmed cases have died.

To date, just over 8 million people in the U.S. have received the J&J vaccine. All the cases were in women; about half of the J&J doses administered in the U.S. were given to women.

Cases have also been reported in Europe with AstraZeneca’s Covid vaccine. The European Medicines Agency said Friday that the condition occurs at a rate of about 1 in 100,000 with the AstraZeneca vaccine. (AstraZeneca’s vaccine is not yet authorized for use in the United States.)

The recommendation to resume use of the Johnson & Johnson vaccine will free up access to nearly 10 million doses that are in the hands of states, territories, and tribal authorities, according to the CDC’s vaccine tracker. The vaccine has been expected to play a significant role in vaccination efforts both in the U.S. and abroad, because of its one-dose regimen and the fact it does not need to be shipped and stored frozen, like the Pfizer and Moderna vaccines.

Some experts have theorized that the events tied to Johnson & Johnson’s vaccine may be part of a “class effect” — that is, a problem one would expect to see in all vaccines made in the same way. It remains unclear whether that’s the case, although the technology used with the J&J vaccine, using a modified adenoviruses, is the same technology used with the vaccines made by AstraZeneca, Russia’s Sputnik V vaccine, and China’s CanSino vaccine.

A number of countries have limited use of the AstraZeneca vaccine after dozens of reports of CVST among recipients. The Gamaleya Center, which makes the Sputnik V vaccine, said in a statement that it had recorded no cases of cerebral venous sinus thrombosis among recipients of its vaccine. It argued that while the adenovirus vectored vaccines share some similarities, each is made differently and “there is no reason and no justification to extrapolate safety data from one vaccine to safety data from other vaccines.”

In a statement to the Hong Kong stock exchange, CanSino said it had not seen any of these cases.

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