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

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?

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.

With the AstraZeneca vaccine. Got it.

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.

And what are platelets?

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.

Got it.

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.

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

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.

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?

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.

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?

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.

Right.

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.

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.

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.

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

Tuesday morning, there is a press conference.

And they present the data.

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

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

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

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?

The effect is really swift.

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

A number of states do the same thing.

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

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

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.

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.

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.

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?

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.

Wow.

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.

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

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.

And what kinds of places would those be?

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

Right.

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.

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?

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

Right.

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.

Right.

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.

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?

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.

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.?

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.

Wow. That’s a very big deal.

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.

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?

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.

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..

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.

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.

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.

Which is a very good thing.

Which is a very, very good thing.

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

Thank you.

We’ll be right back.

Here’s what else you need to know today.

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.

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.

And.

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.

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.

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