Despite weeks of growing vaccinations and good news, headlines about blood clots and a “pause” in deploying the much-anticipated Johnson & Johnson shots have people worried.

Atlantic science writer Katherine J. Wu joins hosts James Hamblin and Maeve Higgins on the podcast Social Distance to explain the situation. Listen to their conversation here:

What follows is a transcript of their conversation, edited for length and clarity:

Maeve Higgins: You wrote about the Johnson & Johnson vaccine pause recently. Could you fill us in on what happened?

Katherine J. Wu: First, to put it in perspective: This was identified very quickly and then addressed very quickly. And that is all a good thing. But basically the situation is: The CDC and FDA reported that they had picked up on six cases of a very specific kind of blood clot in six women under the age of 50. They detected the blood clot in these women within about two weeks of when they had gotten their Johnson & Johnson vaccine. And this is out of nearly 7 million people in the United States who have gotten these vaccines.

So it is a very, very low number of cases so far. More certainly could emerge. But this was enough for federal health officials to say: Hey, this is concerning. We are seeing really the same problem arising in all six of these people. We should stop and take a look. And to be really clear here, what’s going on is a pause. They are literally hitting the pause button here and saying: Let’s get some time, evaluate the data, consult our experts, figure out what’s going on, and critically give doctors the information they need to detect these symptoms in patients and then treat them with the right therapies.

This is not a revocation of the emergency authorization. This is not a withdrawal. This is not like a full recall. This is just: “Let’s wait and see what happens here. Hopefully we’ll have more information to you very soon.”

James Hamblin: Do you have a sense of how confident regulators feel that these clots are indeed related to the vaccine?

Wu: My pulse on the situation is that there’s a moderately confident link at this point. Part of the point of the meetings is to really see how strong that link is, so I can’t conclusively say it, but I think there’s kind of a bump in confidence right now because what they’re detecting with these specific clots looks a lot like what has been going on with the AstraZeneca vaccine, which in terms of vaccine recipe, looks pretty similar to the J&J vaccine. And it’s probably not a complete coincidence that we’re seeing the same side effect appear shortly after both of these very similar-looking vaccines. I think they probably acted more swiftly here with J&J because of what’s already been going on with AstraZeneca.

Hamblin: Yeah, that makes sense. These are both adenoviral vector vaccines, so there are some similarities. AstraZeneca has been associated with clots in Europe, and so it seems like there is a low threshold here. And these specifically were clots in the cerebral veins.

Wu: Yeah, what’s going on here is a very, very specific kind of blood clot. Blood clots happen all the time. These are especially concerning because it’s a super-specific kind of blood clot … appearing in combination with low platelet counts. That’s another linked phenomenon that’s led a lot of researchers to hypothesize something is going on. And that specific combination has raised some eyebrows.

[It’s also] been seen also in response to, ironically, a blood-thinning drug called heparin. And that’s really important to know because heparin is one of the most common blood thinners that doctors use to treat clots. In this specific case, officials are recommending against using heparin because it could potentially make the situation worse. And I think that’s another plus of the pause. Doctors are getting the message loud and clear. Let’s not use it until we really know more and maybe never again for this specific condition.

Hamblin: Right, part of the pause is to help doctors understand what to look for and how to deal with symptoms that might be related to a clot like this. And part of it seems to be messaging to patients who are within this window of having recently had a Johnson & Johnson vaccine. If people experience basically any neurologic symptoms—from dizziness and headaches to anything more significant than that—contact a physician, because it’s going to be especially important to keep a close eye on things like that.

Higgins: [In your piece] you quoted Dr. Angie Rasmussen, an amazing virologist we’ve had on Social Distance before. She got the Johnson & Johnson vaccine nine days ago as we tape, and she’s in the age range of the six women who got the blood-clotting symptoms. So what was her take as a person who got the vaccine and also just as a virologist?

Wu: She had been tweeting about this, and I knew she would provide some really balanced perspective here. First things first: She has gotten the vaccine. There is really no undoing that. But she’s not spending all of her time worrying about getting a clot—partly because that won’t affect whether or not she does—but also because these cases have so far been really, really rare. I don’t think we yet have enough information to say they’re going to appear in exactly X percentage of people, but it’s probably going to be an extraordinarily low number.

Higgins: What are the symptoms to look for in that very rare case?

Wu: There’s a list that the CDC and FDA put out. They said if you develop severe headaches, abdominal pain, leg pain, or shortness of breath within three weeks of your injection, reach out to your health-care provider. But that is going to be the next level up from the expected side effects that some people might get within the first couple of days after getting their injection, which might include a mild headache. So there’s a little bit of subjectivity there, but hopefully people are just keeping an eye out in those three weeks. That’s the critical window we’re looking at here.

Hamblin: Do you think this has been handled correctly? Do you think pausing Johnson & Johnson for everyone was the right move?

Wu: That’s a tricky question to answer, but I think I lean on the side of “yes.” Imagine if the CDC and FDA had waited until it was 20 cases, or 30 cases, or 100 cases … [they] would have gotten questions about why they didn’t pause it when it was just six and we could have prevented 94 more cases.

I think they saw a signal and not just a random cluster of blood clots [that] all looked so similar in this demographic slice of the population. And so they acted quickly. This pause could just be a matter of days to get clarity and make sure that people are aware and looking out for it. I wouldn’t be surprised if more cases come up because we now know to look for it. [But] they are not hoping to permanently derail this vaccine.

Hamblin: Do you think this pause—assuming it doesn’t pan out to be over a major risk factor and Johnson & Johnson gets back online—will, over time, lead to vaccine hesitancy of the sort that we’ve seen with AstraZeneca in Europe? How concerned are you about it exacerbating what we’ve already seen in the U.S.?

Wu: I am concerned about this, but I don’t want to jump to conclusions about how hesitancy will be affected. It’s so tricky because exacerbating hesitancy can occur in a multitude of ways. One is: People hear something negative about the vaccine and they feel less good about taking it. Another is that experts conceal something about the vaccine and the public feels like they’re not being trusted with the relevant information.

The CDC and FDA are in a bit of a “damned if you do, damned if you don’t” situation, but I’ve been really impressed with how a lot of experts have tried to communicate the situation as clearly as they can, and really stress the point that we can make better-informed decisions with this information being publicly out there.

That said, I have spoken to a few people who have already said someone in their life was signed up to get this vaccine and now aren’t feeling as great about getting this one in particular, but they’re also not feeling very good about the other vaccines.

Higgins: I also read your story about second-dose side effects. Our producer Kevin Townsend got his second shot last week, and he was really knocked out. Have you heard from a lot of people who’ve had this tough ride?

Wu: What’s so interesting about this whole vaccine rollout is we’re really coming to grips with just the diversity of the human experience. I’ve talked to people who have had no side effects whatsoever—my mom was actually one of them—[to people like] my husband, who had really bad side effects to both injections. And they are both totally fine. I feel great about both of them being protected.

That said, when there is a second dose, it does seem to, on average, pack a bigger punch. And I think that can be chalked up to the fact that that second dose is a reminder for the immune system about what it’s already seen. You’re just going to rile up the immune system a little more.

The second time you annoy someone, they’re going to yell at you a little louder. And I think that’s roughly equivalent to what’s happening here. I do think it’s a comforting and visible sign that the process is working as expected, but I also don’t think people should worry if they don’t have side effects. Something else that’s comforting is that we did see a lot of side effects documented in the clinical trials. So this was not a surprise. We saw this coming. But the percentage of people who were protected from COVID-19 far exceeded the percentage of people who had tons of side effects. And so, you can have a ton of different experiences in those first few hours after the shot, but don’t read too much into it.

Hamblin: I have a little soreness in my arm, and I take it as evidence that it’s working, that I’m gaining this protection. And the side effects you’re talking about after a second dose are mostly: low energy, fever, aches, chills, occasionally rashes … but they go away pretty quickly. This is nothing like getting COVID-19, nothing like even getting the flu.

Wu: Exactly. It’s like the body reacting to the idea of a pathogen. People have said it’s like having 24 hours of flu-like symptoms, but you’re exactly right—it is not equivalent and certainly not worse than getting COVID-19. The risk calculation here is absolutely clear-cut.