Johnson & Johnson Vaccinations Paused After Rare Clotting Cases Emerge

Federal health officials called for a halt in the use of the company’s coronavirus vaccine while they study serious illnesses that developed in six American women.

Pharmacists prepared syringes with the Johnson & Johnson vaccine in Detroit on Monday. More than seven million people in the United States have received the vaccine.
Credit...Nicole Hester/Ann Arbor News, via Associated Press

WASHINGTON — Injections of Johnson & Johnson’s coronavirus vaccine came to a sudden halt across the country on Tuesday after federal health agencies called for a pause in the vaccine’s use as they examine a rare blood-clotting disorder that emerged in six recipients.

All six were women between the ages of 18 and 48, and all developed the illness within one to three weeks of vaccination. One woman in Virginia died, and a second woman in Nebraska has been hospitalized in critical condition.

More than seven million people in the United States have received Johnson & Johnson shots so far, and another 10 million doses have been shipped out to the states, according to data from the Centers for Disease Control and Prevention.

“We are recommending a pause in the use of this vaccine out of an abundance of caution,” Dr. Peter Marks, the director of the Food and Drug Administration’s Center for Biologics Evaluation and Research, and Dr. Anne Schuchat, the principal deputy director of the C.D.C., said in a joint statement. “Right now, these adverse events appear to be extremely rare.”

While they framed the move as a recommendation to health practitioners, the impact was immediate. By Tuesday evening, every state, the District of Columbia and Puerto Rico had announced a pause in Johnson & Johnson vaccine injections.

The same went for the U.S. military, federally run vaccination sites, and CVS and Walgreens, two pharmacy giants that participate in the federal program, officials said. Rite Aid, Walmart and Publix also announced that they had paused Johnson & Johnson injections.

Beyond American shores, Johnson & Johnson said it would delay the rollout of its vaccine in Europe, where several countries were poised to start administering it this week. South Africa, devastated by a more contagious variant of the virus that emerged there, also suspended use of the vaccine. Australia announced it would not purchase any doses.

The Biden administration tried to portray itself as well prepared for the setback. President Biden said he would meet his goal to have enough doses to vaccinate every American adult who wanted it by the end of next month.

“There is enough vaccine, that is basically 100 percent unquestionable, for every single, solitary American,” he insisted.

The reaction prompted an intense debate among public health experts about whether guarding against such a rare disorder was worth the cost. Scores of vaccine appointments were canceled this week, and some public health officials feared that by fueling vaccine hesitancy and conspiracy theorists, the pause could prompt fewer Americans to get vaccinated — and expose them to far more risk.

Others said the F.D.A. and the C.D.C. simply had no choice.

“It’s incredibly challenging, but to ignore it would have been worse,” said Rachael Piltch-Loeb, an expert in health risk communication at the N.Y.U. School of Global Public Health. If the public suspected that the government was concealing serious potential side effects, she said, far more people might decide against vaccination.

Dr. Janet Woodcock, the acting commissioner of the F.D.A., said at a news conference that the pause might last only “a matter of days,” although she said that depended on “what we learn in the next few days.” Dr. Schuchat said the pause was enacted partly to “prepare the health care system to recognize and treat patients appropriately.”

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Listen to ‘The Daily’: The Johnson & Johnson Vaccine Pause Explained

Scores of appointments were canceled after the U.S. called for a halt on the use of the Johnson & Johnson shot. Here’s the story behind that decision.
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Listen to ‘The Daily’: The Johnson & Johnson Vaccine Pause Explained

Hosted by Michael Barbaro; produced by Rachel Quester, Jessica Cheung and Leslye Davis, with help from Alexandra Leigh Young; edited by MJ Davis Lin; and engineered by Chris Wood.

Scores of appointments were canceled after the U.S. called for a halt on the use of the Johnson & Johnson shot. Here’s the story behind that decision.

michael barbaro

From The New York Times, I’m Michael Barbaro. This is “The Daily.”

[music]

Today, the decision-making process that led the U.S. government to suspend the use of Johnson & Johnson’s COVID-19 vaccine, how long it may last and the impact it could have. I spoke with my colleague, science reporter Carl Zimmer. It’s Thursday, April 15th.

Carl, walk us through how this very big decision was reached to pause vaccinations with Johnson & Johnson’s vaccine in the U.S. Where does that story start?

carl zimmer

The story starts in Europe with another vaccine, one that’s made by AstraZeneca. AstraZeneca is going into widespread use in Europe. It’s not yet authorized in the United States. And after a while, the European regulators notice a few rare cases of blood clots, which start to get them concerned. And so they begin to investigate whether that has anything to do with the vaccine.

michael barbaro

With the AstraZeneca vaccine. Got it.

carl zimmer

So blood clots are a surprisingly common thing. Thousands of people get blood clots every day. So just because people who get vaccinated get a blood clot, that doesn’t really tell you anything in particular about whether the vaccine is dangerous. You have to take a closer look. And when European regulators took a closer look at people getting vaccinated, they noticed that there was a small amount of people who were all getting the same unusual kind of blood clot. They were taking place in the brain, and they were associated with very low levels of something called platelets.

michael barbaro

And what are platelets?

carl zimmer

Platelets are a component of the clotting process in our bloodstream. So when we get little wounds in our blood vessels, platelets jump into the wound and basically create a little patch. And that allows us to heal. So that’s good. What’s bad is when the platelets start to form a big clot, which can then break loose and go around in our bloodstream and end up in dangerous places, like in our brains.

michael barbaro

Got it.

carl zimmer

And so what puzzled and concerned European regulators is that you had this very strange condition where you had a combination of these particular kinds of clots with low platelets. So they thought, maybe we’re looking at a rare but distinct and serious condition. Now, it’s not clear why, in these people, they’re getting brain clots along with low platelets. One idea is that all the platelets in the blood are going into these clots, and so their level of platelets is going down. This is a hypothesis that’s coming out of the first studies of people who have gotten vaccinated with AstraZeneca and have developed these peculiar blood clots with the low platelets.

michael barbaro

OK, so how do we get to Johnson & Johnson?

carl zimmer

European regulators start to wonder if they could get some clues to what’s happening with AstraZeneca by looking at the Johnson & Johnson vaccine because it’s similar, in some ways, to AstraZeneca. They use the same basic technology, unlike Moderna and Pfizer. So last week, they start to look at data from the United States. Now, at first, there isn’t a lot of data that really points to something similar happening with Johnson & Johnson, but over the weekend, that changed. By the weekend, the C.D.C. and the F.D.A. have gotten six reports that are very similar to what European regulators were seeing with AstraZeneca, people who have suffered from this distinctive brain clot combined with low platelets. Six people out of seven million or so who have gotten the Johnson & Johnson vaccine, that’s tiny. But it’s a pretty unusual combination. Also, all of those six people were women, and one of those women actually died of her blood clot. And so that started to set off alarms in the government that maybe we need to stop and take a close look at what’s happening.

michael barbaro

OK, and so how do U.S. regulators, who have been watching, I’m sure, what was happening in Europe with AstraZeneca, how do they start to think about how to respond to this very small — six — number of cases of blood clots in people who have gotten the Johnson & Johnson vaccine?

carl zimmer

Once these six cases had come to light, government officials got together and talked about what their options were. And basically, it came down to two. On the one hand, they could issue a warning. In other words, they could let doctors know that they had seen this incredibly rare but very serious condition, and just have doctors be aware that this is an issue, and maybe keep an eye out for it. Or they could recommend a pause on vaccination. In other words, they would recommend that doctors and vaccination centers and so on stop for now giving out the Johnson & Johnson vaccine while they investigated it further.

michael barbaro

Wow. Those are very different options. I mean, one is essentially send the medical world a memo saying heads up. The other is far more significant. It’s stopping temporarily the use of the Johnson & Johnson vaccine, which of course would be very disruptive. So what factors are influencing how they are considering these two options?

carl zimmer

One important factor is that they were wondering how many more cases out there ar ethere. So they knew of six, but maybe there were more that people just hadn’t reported yet.

michael barbaro

Right.

carl zimmer

And so if they just give a quiet little warning, people might not pay attention to it. But if they say, hey folks, we’re taking this seriously, that might spur doctors and patients and so on to get in touch and say, hey, guess what, I’ve seen the same thing, or I’ve experienced the same thing. It’s also of concern because there’s some indication that some of the standard way to treat a blood clot, with a drug called heparin, might actually be bad for this particular condition. So you really want to make sure that people are aware that there is a potential risk of just treating a blood clot like any other blood clot.

michael barbaro

That’s interesting. So these regulators are not just worried about the danger of these blood clots, but also about the danger of potentially mistreating them.

carl zimmer

They’re concerned that this might be a risk. You know, again, they have to figure out what’s going on. You know, they really have to figure out if this is even a cause and effect that they’re looking at. But these kinds of concerns shifts the government officials away from just sending out a warning to actually going for a pause.

michael barbaro

Right. And of course, a pause is what the regulators decide on. So how do they describe that decision to the American public?

archived recording

Thank you. And thank you all for joining us.

carl zimmer

Tuesday morning, there is a press conference.

archived recording

This morning, the F.D.A. and C.D.C. announced that, out of an abundance of caution, we’re recommending a pause in the use of the Johnson & Johnson COVID-19 vaccine. With the acting director of the Food and Drug Administration, Janet Woodcock, and other officials. These events appear to be extremely rare.

carl zimmer

And they present the data.

archived recording

One case was fatal, and one patient is in critical condition. Right now, these events appear to be extremely rare. That said, COVID-19 vaccine safety is a top priority for the federal government.

carl zimmer

Said that this is very, very rare, but on the other hand, it’s serious enough that they have to investigate it.

archived recording

We are committed to an expeditious review of the available information.

carl zimmer

And expected that this investigation might take only a few days.

archived recording

The time frame will depend, obviously, on what we learn in the next few days.

carl zimmer

And during those few days, they recommended that the Johnson & Johnson vaccine be paused.

archived recording

This concludes today’s media briefing.

michael barbaro

And Carl, you just used an important word, recommended. This is federal guidance. It is not a mandate. So what is the immediate response to U.S. health regulators giving the recommendation that the Johnson & Johnson vaccinations be paused?

carl zimmer

The effect is really swift.

archived recording

The F.D.A. and the C.D.C. has announced that they are now recommending a pause. Walgreens, CVS and Kroger among the national pharmacies that also will pause using J&J as well.

carl zimmer

Walgreens and CVS both quickly say that they’re going to pause using Johnson & Johnson vaccines.

archived recording

You might as well view it as a pause nationwide.

carl zimmer

A number of states do the same thing.

archived recording

Signs now flash this message outside the federally supported vaccine site — no J&J here.

carl zimmer

There’s a huge wave of cancelations of appointments to get vaccinated.

archived recording

It’s so rare, the side effects, that I really wasn’t worried about it.

carl zimmer

There are mobile vaccination clinics that are just canceled in different cities.

archived recording

I’m just disappointed there’s no shot at all. So I’m definitely disappointed.

carl zimmer

So this has gigantic effects immediately. And by the end of Tuesday, Johnson & Johnson vaccination across the United States has pretty much ground to a halt.

michael barbaro

Right. And in a sense, what that tells us is that the pause worked. Right? I mean, pharmacies and states do exactly what the C.D.C. and the F.D.A. have guided them to do.

carl zimmer

Yes. So the pause is happening in exactly the way that government officials had hoped for, but that doesn’t mean that there aren’t some concerns about what the long-term effects could be of this pause.

[music]

michael barbaro

We’ll be right back.

Carl, once this pause is in effect, it raises tons of questions. So let’s spin through those questions. The first is, what will be the consequence of the pause on the U.S. vaccination campaign, on the deadlines and the schedule for returning to normalcy?

carl zimmer

Johnson & Johnson has been a crucial component of the Biden administration’s plans for getting the United States back to normal. They were supposed to be delivering 100 million doses by the end of May.

michael barbaro

Wow.

carl zimmer

And they’ve already been having some problems with manufacturing their vaccines, and had to essentially throw out 50 million doses. So now, even pausing for a few days can throw that plan into even further uncertainty.

michael barbaro

Right. So on a logistical, scheduling level, this is a meaningfully big deal.

carl zimmer

It’s a big deal, especially in parts of the country where the other two vaccines, by Moderna and Pfizer, are not so easy to give out.

michael barbaro

And what kinds of places would those be?

carl zimmer

Well, Moderna and Pfizer’s vaccines both need to be frozen.

michael barbaro

Right.

carl zimmer

Johnson & Johnson’s is a lot easier to use. You just throw it in the fridge. And so you can have these mobile clinics driving around with them. You can bring them to rural areas, where they don’t have the facilities for keeping frozen vaccines. I mean, you can do things with the Johnson & Johnson vaccine that you just couldn’t do with the other vaccines. So for example, there are people who can’t get to a clinic. They’re homebound. And so if you’re going to send out public health workers to get to them with the vaccine, you don’t want them to have to be lugging around big freezers. It’s really easy, relatively speaking, to send them out with the Johnson & Johnson vaccine, going to people’s homes and vaccinate them. And do it once, and then you’re done. And it also had the advantage of just being one shot. And so a lot of people just really liked the prospect of getting just one dose, as opposed to two with Moderna and Pfizer. So there were going to be a sizable number of people who were going to really benefit from having this option of Johnson & Johnson. And now, for the next few days at least, that’s not an option.

michael barbaro

Carl, what about hesitancy? What might this pause do to and for a U.S. public that already showed signs in some demographics, in particular, of being skeptical of COVID vaccines, despite data, as we’ve talked to you about many times, that shows how historically safe and effective all three of these vaccines are?

carl zimmer

Well, if people just hear a few words, they might get concerned. They might hear vaccine, blood clot, oh my gosh, that’s bad.

michael barbaro

Right.

carl zimmer

So really, we’re going to have to see how well the U.S. government handles the messaging of what they’re doing in the days to follow, because what’s crucial for people to understand is that, first of all, this is so far an incredibly rare condition. If you look at 7 million people who have gotten this vaccine so far in the United States, and we only know of six cases of this particular condition, you know, that means that 99.999999% of people who got the vaccine didn’t get that condition.

michael barbaro

Right.

carl zimmer

So if you’re thinking about your odds, well, your odds of getting COVID are way higher right now, and the risks you face from COVID of ending up in the ICU and potentially dying are also, by comparison, incredibly high. So this is going to be the kind of information that public health officials are going to have to get out, and get out clearly, in the days to follow.

michael barbaro

What you’re describing is essentially a messaging battle for the government, how easily can it communicate safety and the rarity of these blood clots. Do you have any sense yet, based on your reporting, of how the government is thinking of approaching the question of communicating that?

carl zimmer

I think that the theme that the government is hammering on over and over again is that this is how government regulators keep us safe, that they are keeping a very careful eye on these new vaccines, and at even the slightest suggestion of a problem, they’re are going to take it very seriously and look very closely at it. That’s the message.

michael barbaro

Carl, so far we have been talking about the domestic impacts of this decision on Johnson & Johnson’s vaccine, but every vaccine maker is attempting to put doses into arms across the world. So is the U.S. government. We all want as many people to be vaccinated as possible in every country. So how, if at all, does this pause impact people outside the U.S.?

carl zimmer

The ramifications of these findings are pretty big right now. Johnson & Johnson was in the process of seeking authorization in Europe for their vaccine. They’re putting that on pause for now. There were such concerns in Australia that they decided that they’re not going to buy Johnson & Johnson vaccines at all.

michael barbaro

Wow. That’s a very big deal.

carl zimmer

It is. Johnson & Johnson had promised half a billion doses to an effort called COVAX to get vaccines to lots of countries that might not be able to afford to get them on their own. This development throws that into question. It’s not clear what COVAX is going to do about this. South Africa had been planning on using AstraZeneca to treat their health care workers. Then it turned out that AstraZeneca might not be effective against their particular variant of COVID, and so they said that’s OK, we’ll use Johnson & Johnson. Well, they’re probably not going to use Johnson & Johnson for now. So this is having impacts across the world.

michael barbaro

So given just how high the stakes are, here and globally, for resuming J&J vaccinations, do you have any sense of how long this pause may last?

carl zimmer

If it follows the example of what happened in Europe with AstraZeneca, I think this will be resolved in a few days. I think what’s happening now is that the F.D.A. and the C.D.C. are taking in new reports that might or might not represent more of these cases of these peculiar blood clots, and then they’re going to look at the data and figure out, well, what do we do next? Is this as significant a risk that we need to adjust how we give the vaccines out? One possibility is that they might restrict who gets the vaccine, simply to minimize risk. So far, the six cases are in women between 18 and 48. Does that mean that this vaccine represents a very small, but significant risk specifically for women? We don’t know yet. It could be that a bunch of cases involving men might come in today and tomorrow and in the next few days. We’ll see. But you could imagine decisions based on age or on sex or other things that might come out in the next few days.

michael barbaro

Interesting. So this could result in an unpausing of the vaccine pretty quickly, but one where there are now restrictions and different guidelines about who can get the Johnson & Johnson vaccine in the U.S..

carl zimmer

Yes. And what people have to recognize is that pauses happen all the time in the world of vaccines and drug development. There’s a huge amount of attention on this particular pause because we’re in a pandemic, right? But you just have to bear in mind that pauses happen, and they happen a lot. And then generally what happens is that the pause ends and things go forward.

michael barbaro

That said, Carl, if you’ve just gotten the J&J vaccine, this pause period is one where you’re craving information. And I wonder what guidance, based on your reporting, should be given to people in that position.

carl zimmer

First off, if you’ve just gotten the Johnson & Johnson vaccine, take a deep breath and remember that this condition seems to be incredibly rare. It may or may not have anything to do with getting this vaccine in the first place. Then just monitor your health. The F.D.A. and the C.D.C. have given a list of symptoms, including severe headache, for example. So if, in the three weeks after you get your Johnson & Johnson vaccine, you experience a severe headache, maybe a pain in your leg, call your doctor. Let your doctor know that you’re not feeling great, and tell them that you got the Johnson & Johnson vaccine. And then you and your doctor can take things from there. The chances are overwhelmingly likely that you’re not going to have these problems, that you’re not going to be calling your doctor about it, and you’re going to come out of this protected from COVID.

michael barbaro

Which is a very good thing.

carl zimmer

Which is a very, very good thing.

michael barbaro

Well, Carl, thank you. We, as always, appreciate your wisdom.

carl zimmer

Thank you.

[music]

We’ll be right back.

Here’s what else you need to know today.

archived recording (joe biden)

I’m now the fourth United States president to preside over American troop presence in Afghanistan. Two Republicans, two Democrats. I will not pass this responsibility on to a fifth.

michael barbaro

In a speech on Wednesday, President Biden explained his decision to withdraw all American troops from Afghanistan by September 11th, saying that the United States had achieved its goal of ensuring that Afghanistan would never again serve as a launching pad for a terrorist attack on the United States.

archived recording (joe biden)

I’ve concluded that it’s time to end America’s longest war. It’s time for American troops to come home.

michael barbaro

Biden said that the drawdown would begin on May 1st, the deadline for withdrawal set by his predecessor, Donald Trump. But in a pointed warning, Biden admonished the Taliban against seeking to attack U.S. forces as they left Afghanistan.

archived recording (joe biden)

And the Taliban should know that, if they attack us as we draw down, we will defend ourselves and our partners with all the tools at our disposal.

michael barbaro

And.

archived recording (david fowler)

So in my opinion, Mr. Floyd had a sudden cardiac arrhythmia, or cardiac arrhythmia due to his atherosclerotic and hypertensive heart disease, or —

michael barbaro

On Wednesday, defense lawyers in the murder trial of Derek Chauvin sought to rebut claims that George Floyd died because Chauvin deprived him of oxygen, calling to the stand a former medical examiner for Maryland, Dr. David Fowler, who said that Floyd’s heart condition, drug use, and even carbon monoxide from the tailpipe of a nearby car contributed to his death.

archived recording (david fowler)

All of those combined to cause Mr. Floyd’s death.

michael barbaro

As medical examiner in Maryland, Fowler had previously ruled that the 2018 death of another Black man in police custody, Anton Black, was an accident, prompting a lawsuit from Black’s family. Finally, prosecutors in Minnesota said they would charge the white police officer who fatally shot Daunte Wright, a 20-year-old Black man, with second-degree manslaughter. Police officials have said that the officer, Kim Potter, appeared to have shot Wright by accident after mistaking her gun for her taser. Today’s episode was produced by Rachel Quester, Jessica Cheung, and Leslye Davis, with help from Alexandra Leigh Young. It was edited by M.J. Davis Lin, and engineered by Chris Wood.

[music]

That’s it for “The Daily.” I’m Michael Barbaro. See you tomorrow.

Scientists with the F.D.A. and the C.D.C. will jointly examine possible links between the vaccine and the disorder and determine whether the F.D.A. should continue to allow emergency use of the vaccine for all adults or modify the authorization, possibly by limiting the vaccine to certain population groups. An emergency meeting of the C.D.C.’s outside vaccine advisory committee has been scheduled for Wednesday.

Despite Mr. Biden’s assurance, the pause will complicate the nation’s vaccination efforts at a time when many states are confronting a surge in new cases and seeking to address vaccine hesitancy. Regulators in Europe and elsewhere are concerned about a similar issue with another coronavirus vaccine, developed by AstraZeneca and Oxford University researchers.

At the news conference, Dr. Marks drew a link between the two vaccines, saying the clotting cases were “very, very similar.” The vaccines are based on similar technology, but AstraZeneca’s has not yet been authorized for emergency use in the United States.

The vast majority of the nation’s vaccine stock comes from two other manufacturers, Pfizer-BioNTech and Moderna. Those two vaccines use a different technology than Johnson & Johnson’s and AstraZeneca’s. More than 180 million doses of Pfizer and Moderna have been administered, according to the C.D.C.’s latest statistics, and federal officials stressed on Tuesday that they had seen no evidence of the blood clots that led to the pause of the Johnson & Johnson vaccine — or of any other significant safety concern.

The decision is a fresh blow to Johnson & Johnson. Late last month, the company discovered that workers at a Baltimore plant had accidentally contaminated a batch of its vaccine, forcing the firm to throw out millions of doses. With federal certification of that factory in doubt, Johnson & Johnson’s shipments dropped to one-fourth or less of what had been expected, a severe disappointment to White House and state officials.

Jeffrey D. Zients, the White House coordinator of the pandemic response, said that the federal government was still shipping out 28 million Pfizer and Moderna doses this week and that about three million shots were being administered daily.

But the White House had been hoping for an even better showing. At one point this spring, officials were planning on weekly shipments of more than four million doses of Johnson & Johnson vaccine beginning this month.

Because Johnson & Johnson is one dose and easily stored, it was destined for specialized vaccine outreach programs. In Colorado and California on Tuesday, mobile vaccine clinics in rural areas were canceled. In Chicago, vaccination events for restaurant employees and aviation workers were postponed indefinitely. At colleges in Ohio, New York and other states, where the one-dose vaccine offered a chance to quickly inoculate students before they left campus for summer, appointments were called off en masse.

With only two vaccines instead of three, federal officials expect to have enough doses to cover at most 230 million adults by the end of May, roughly 30 million shy of the total adult population. But despite public campaigns to convince them, a certain share of adults are expected to refuse shots, so that supply may cover all the demand.

Government experts are concerned that doctors may not be trained to spot or treat the rare disorder if recipients of the vaccine develop symptoms. Dr. Marks said that a standard treatment for blood clots — use of an anticoagulant drug called heparin — “can actually cause tremendous harm, or the outcome can be fatal.”

The C.D.C. and the F.D.A. recommended that people who have received the Johnson & Johnson vaccine within the past month contact their doctors if they experience severe headaches, abdominal pain, leg pain or shortness of breath. Officials said the most common symptom of the disorder was a persistent, moderate to severe headache that begins six days or later after the shot.

Dr. Schuchat, the C.D.C. official, said the risk of dangerous blood clots was “very low” for people who received Johnson & Johnson’s vaccine more than a month ago.

In a statement, Johnson & Johnson said it supported “open communication” with health care professionals and the public and had been working closely with medical experts and health authorities, including in Europe. “We have made the decision to proactively delay the rollout of our vaccine” there, the firm said.

The company also said that it was pausing vaccinations in its clinical trials. It is currently testing the effectiveness of two doses, as opposed to the single dose that was authorized in February.

In the United States alone, 300,000 to 600,000 people a year develop blood clots, according to C.D.C. data. But the six cases that led to the pause involved a rare combination of symptoms. The clots occurred in the brain, in a condition called known as cerebral venous sinus thrombosis. The clots were accompanied by low levels of platelets, a component of blood that helps form clots that normally help heal wounds.

All of the women developed the illness within about one to three weeks of vaccination, with a median time of nine days. The hospitalized Nebraska woman is in her late 40s and developed blood clots two weeks after her shot, state health officials said.

The Virginia woman, 45, developed symptoms six days after she was inoculated in early March and died six days after that, health officials said. They said that the public attention is already generating a spate of new reports of possible cases to federal authorities.

Federal officials said there was broad agreement in the senior ranks of the administration about the need to pause use of the vaccine while they investigate whether the blood clots are linked to an immune system response generated by the vaccine.

Dr. Marks said the federal government had not ordered the vaccine’s suspension because health providers may decide that for a particular patient, the benefits of a shot outweigh the risks. “We’re not going to stop that provider from administering the vaccine because it could be right,” he said.

The concerns about Johnson & Johnson’s vaccine mirror those about AstraZeneca’s. European regulators began investigating that vaccine last month after some recipients developed blood clots and low platelet counts like those of the Johnson & Johnson recipients.

Out of 34 million people who received AstraZeneca’s shots in Britain, the European Union and three other countries, 222 experienced blood clots linked to a low level of platelets. A majority of these cases involved women under 60.

On April 7, the European Medicines Agency, the main regulatory agency, concluded that the disorder was a very rare side effect. The European regulators argued that the benefit of the AstraZeneca vaccine vastly outweighed that small risk. Countries in Europe and elsewhere have continued to give the vaccine to older people, who are more at risk from the coronavirus, while restricting it in younger people.

Both Johnson & Johnson and AstraZeneca use adenoviruses to carry DNA into human cells to begin the process of generating immunity to the coronavirus. It is not yet known whether that shared technology causes clotting in rare cases.

Researchers have speculated that DNA carried by the adenoviruses may set off the immune reaction in some people. The condition may be very rare because people have to have some trait — still unknown — that predisposes them to this malfunction.

The first public sign of concern about Johnson & Johnson’s vaccine came on April 9, when the European Medicines Agency announced that it was investigating reports of four cases of blood clots in people who received the vaccine in the United States. One case occurred in the clinical trial that took place before the vaccine was authorized. Three occurred in the vaccine rollout. One of them was fatal, the agency said. The regulators said the cluster of cases required investigation, but that it was not clear whether the vaccine was to blame.

Reporting was contributed by Sheryl Gay Stolberg, Michael D. Shear and Eileen Sullivan from Washington; Jan Hoffman, Michael Gold and Giulia McDonnell Nieto del Rio from New York; Mitch Smith from Chicago; Monika Pronczuk from Brussels; Lynsey Chutel from Johannesburg; and Rebecca Robbins from Bellingham, Wash.