C.D.C. Panel Keeps Pause on Use of J&J Vaccine, Citing Need to Assess Potential Risks

An advisory committee debated the very few cases of a rare blood disorder and worried about the suspension’s effect on global needs for a one-shot, easy-to-ship vaccine.

Doses of the Johnson & Johnson Covid vaccine at a clinic in Los Angeles last month.
Credit...Mario Tama/Getty Images

The pause in the use of Johnson & Johnson’s Covid-19 vaccine may continue for a week to 10 days, after expert advisers to the Centers for Disease Control and Prevention determined on Wednesday that they needed more time to assess a possible link to a rare but serious blood-clotting disorder.

The decision not to reinstate the vaccine has painful consequences, nationally and globally. It may further erode public confidence in vaccination in general and slow the rollout of desperately needed shots to rural and underserved areas and homebound people. The vaccine is considered ideal for hard-to-reach people and places because it requires only one shot and is more easily stored and shipped than the vaccines made by Moderna and Pfizer-BioNTech, which must be kept at very low temperatures.

“Putting this vaccine on pause, for those of us that are frontline health care workers, has really been devastating,” Dr. Camille Kotton of Harvard Medical School told the panel. She said that losing the Johnson & Johnson vaccine even temporarily represented a big blow to efforts to stop the pandemic, especially in underserved communities.

The pause, first recommended on Tuesday by U.S. health officials, led Johnson & Johnson to delay its rollout of the vaccine in Europe, where several countries were poised to start administering it this week. The continent is reeling from the fallout over rare cases, sometimes fatal, of a similar blood disorder that has prompted several nations to limit or reject the widespread use of the AstraZeneca-Oxford vaccine and recommend alternatives.

South Africa, devastated by a worrisome variant of the virus that emerged there, also suspended use of the Johnson & Johnson vaccine. Australia announced it would not purchase any doses. And the European Union indicated that it would consider new deals only with companies that were not using the technology employed by the Johnson & Johnson or AstraZeneca vaccines.

Noting that many countries follow the United States’ lead when it comes to vaccines, Dr. Grace Lee of Stanford University, an expert on the panel, said that although the committee’s responsibility was to the United States, “I also feel the weight of the burden of a global responsibility that we also have and the impact that our decision-making can potentially worsen inequities.”

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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.

The advisory group’s emergency meeting on Wednesday was called to review the reason for the pause: six cases of rare and severe blood clots in the brain in women ages 18 to 48, one of whom died. All of the women had received the Johnson & Johnson vaccine before developing the clots, although it is unclear whether the vaccine was responsible. In addition, the panel learned of the clotting problem in a seventh woman who received the vaccine after it was authorized, and in a man who received it during its clinical trials.

As of Tuesday, more than seven million people in the United States had received the Johnson & Johnson shot, representing about 5 percent of vaccinations nationwide.

Advisory meetings usually end with a vote on whether or how to use a vaccine. But in this case, the panel members declined to vote after reviewing several options — including whether to limit use of the vaccine to older adults, as many European nations have done — saying that they did not have enough information to assess the potential risks.

Doran Fink, an official of the Food and Drug Administration, proposed a different strategy to the panel, suggesting that the vaccine could go back into use while researchers continued to study the potential risk. Doctors and patients could be provided with information about the findings so that patients could consider whether to accept the vaccine. “Our current thinking is that this risk could be managed by inclusion of warning statements,” Dr. Fink said.

But some experts on the C.D.C. panel leaned in other directions, fearing that more patients could be harmed if vaccinations resumed without a full understanding of the potential risks. One warned that the rare condition could cause long-lasting neurological damage. Others reminded their colleagues of the risk from Covid itself, and the message the panel would send if it prolonged a suspension in the use of a one-and-done vaccine.

The Advisory Committee on Immunization Practices, or ACIP, is a panel of independent experts who advise the C.D.C. on its vaccine policies.

The clotting disorder of concern in the vaccine recipients is different — and much rarer — from typical blood clots, which develop in hundreds of thousands of people every year. The seven women experienced not only clotting in the brain but a notably low level of platelets, parts of the blood that help form normal clots in response to an injury.

None of the women had recently given birth, which can increase the risk of more common blood clots, and only one was known to be taking hormonal treatment. So far, there is no evidence that birth control pills, which can also raise the risk of blood clots, were involved.

Three of the women had large, dangerous clots in other parts of their body, not just in the brain.

The rare combination of severe clots and low platelets stood out to experts as a safety signal. Why it develops is not known, and so far there is no way to predict whether an individual is susceptible.

The condition is very similar to one linked to the AstraZeneca Covid vaccine, also affecting some relatively young women in Europe, where that vaccine was in wide use. Researchers in Germany and Norway found that patients there had developed antibodies that activated their platelets — an aberrant response to the vaccine — setting off a cascade of clotting and bleeding. Specialized blood tests can detect the antibodies to confirm the diagnosis, and some of the U.S. patients tested positive. Not all were tested.

Both AstraZeneca and Johnson & Johnson use adenoviruses to ferry DNA into human cells to create immunity against the coronavirus. Researchers suspect that some aspect of that technology plays a role in the blood disorder. But they also emphasize that because the condition is so very rare, some quirk of biology very likely predisposes certain people to have a bad reaction to the vaccine. If the vaccine alone were responsible, there would be many more cases, some researchers say.

The vaccines made by Moderna and Pfizer-BioNTech employ genetic material called mRNA and do not use viruses.

Much of the world had been counting on the AstraZeneca vaccine to fight the pandemic, but many countries have now restricted its use to adults older than 30 or 50, and some have stopped using it altogether. Some people who are still eligible for it are declining it out of fear. AstraZeneca’s vaccine has not been authorized in the United States, although the company is expected to apply for permission to distribute it here.

Use of the Johnson & Johnson vaccine in the United States began on March 2, and the first case of blood clots in the brain was reported on March 19. About 1.4 million women ages 20 to 50 — the age range of those who had the clots — have received the vaccine.

The panel of experts discussed the known background rates of low platelets and of the clots in the brain — known as central venous sinus thrombosis — in the general population and in younger women like those who had the clots, but noted that there was not enough data to precisely estimate how often the two conditions occurred at the same time.

But based on rough estimates, the clotting disorder in women ages 20 to 50 who received the Johnson & Johnson vaccine occurred at least three times more often than would be expected, according to Dr. Tom Shimabukuro, a safety expert from the C.D.C.

In the coming weeks, he said, the rate would become clearer as more reports arrived. “We’ll get a better picture of what’s going on.”

“Right now, we believe these events to be extremely rare, but we are also not yet certain we have heard about all possible cases, as this syndrome may not be easily recognized as one associated with the vaccine,” Dr. Rochelle P. Walensky, the C.D.C. director, said at a White House news conference on the pandemic on Wednesday.

During the panel discussion, experts noted that the “risk window” for the condition among vaccine recipients was still open and that new cases might emerge, because nearly 3.8 million people had received the shot within the last two weeks. In the six women, the severe clotting developed within about two weeks of the shot.

Other experts encouraged dissemination of health information on diagnosis and treatment of the condition so that awareness would spread among doctors, emergency rooms and people who had received the vaccine. A key point is that the blood-thinner heparin, a common treatment for clots, can harm these patients and should not be used.

Officials also noted that people with the condition needed to be treated as soon as possible because the clots were so serious. Some patients needed invasive procedures to remove large clots from blood vessels in their brains.

Several panel members reiterated that two other vaccines — from Moderna and Pfizer-BioNTech — are available, neither associated with the clotting problem, so continuing the pause would not stop most people in the United States from being vaccinated.

At the news conference, Jeffrey D. Zients, the White House’s pandemic coordinator, said that the pause would not generally interrupt the momentum of the country’s vaccination campaign.

“In the very short term, we do expect some impact on daily averages as sites and appointments transition from Johnson & Johnson to Moderna and Pfizer vaccines,” he said. “We have more than enough Pfizer and Moderna vaccine supply to continue or even accelerate the current pace of vaccinations.”

Public health experts have repeatedly emphasized that the clotting disorder is rare and that the benefits of the AstraZeneca and Johnson & Johnson vaccines far outweigh their risks. But when an adverse effect has the potential to be devastating or fatal — like blood clots in the brain — some regulators and segments of the public consider the risk unacceptable, even if it is extremely rare.

The safety bar for vaccines is set high because they are given to healthy people. The seemingly greater vulnerability of younger people to the clotting disorder is of particular concern, because their risk of severe illness from Covid itself is lower than that in older people. Those differences suggest that over all, compared to older people, younger people may have less to gain and more to lose from the Johnson & Johnson and AstraZeneca vaccines.

Reporting was contributed by Noah Weiland, Madeleine Ngo and Virginia Hughes.