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Six weeks and nearly seven million shots into the rollout of Johnson & Johnson’s single-dose Covid-19 vaccine, federal health agencies have recommended pausing its distribution, following six reported cases of a rare, but serious type of blood clot as a side effect. On the morning of April 13th, the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) issued a joint statement announcing the distribution hiatus “out of an abundance of caution” in order to review data from six women in the United States between the ages of 18 and 48 who developed the blood clotting disorder within two weeks of receiving the vaccination. One has died, and another has been hospitalized in critical condition, the New York Times reports.

Within a matter of hours, all 50 states — as well as Washington, D.C. and Puerto Rico — had implemented guidelines following the federal recommendation to suspend use of the Johnson & Johnson’s Covid-19 vaccine. And while FDA and CDC noted that the blood clot side effects “appear to be extremely rare,” the pause is (understandably) concerning to many — including the approximately seven million people who had already received the New Jersey-based pharmaceutical company’s vaccine.

But beyond the immediate concerns over the vaccine’s safety, there’s also the question of whether this recommendation could have a ripple effect on the current trajectory of the pandemic — including its effects on the already vaccine hesitant, and whether this could mean a longer road to herd immunity. Here’s what to know.

Don’t panic: This isn’t unusual or unexpected

At this point in the pandemic — when it looks like we may be turning a corner thanks to the increasing availability of Covid-19 vaccines — the announcement that the distribution of the Johnson & Johnson/Janssen vaccine was put on pause was jarring. But if you look beyond the alarming headlines and misinformed social media posts, it becomes clear that this type of action is a normal part of the process. 

“Definitely don’t panic,” says Dr. Wilbur Chen, the chief of the adult clinical studies section within the Center for Vaccine Development and Global Health at the University of Maryland School of Medicine. “We’re doing this only because we want to make sure that safety is the primary factor in all this. And how do we ensure safety? We report [potential side effects] as accurately and quickly as possible.” 

From here, Chen says that medical professionals and researchers will continue to monitor all three of the vaccines on the market for safety signals. “This is not unexpected,” he tells Rolling Stone. “I didn’t predict that it was going to be this event, but it could have been any number of things with any vaccine or any therapeutic.”

In fact, this means the research and safety monitoring processes are working

So why wasn’t the blood clot side effect discovered during the clinical trials? It comes down (at least in part) to numbers. Out of the nearly seven million people in the United States who received the Johnson & Johnson Covid vaccine, at this point, there are six reported cases of the side effect — making the odds of experiencing it less than one in a million. The Johnson & Johnson phase 3 clinical trials had approximately 40,000 participants. 

“Clinical trials are not designed to be able to evaluate these extremely rare side effects,” Chen explains. “And that’s why, for all drugs and vaccines that it approves, the FDA has a commitment for post-marketing safety surveillance. That’s what we’re seeing now, and that’s what we have seen for all the vaccines and drugs that have been approved in the past.” 

If anything, the pause is an indication that the system is working. “The medical system didn’t fail,” says Dr. Andrew Eisenberger, a hematologist at Columbia University Irving Medical Center. “We needed vaccines quickly, and we basically had to accelerate the approval process. That wasn’t a failure: we had to do that because there was a once-in-a-century pandemic going on. I would actually say that the system really succeeded. We can’t entirely prevent something like this from happening, but we picked it up very quickly.”

And no, this doesn’t mean that we should have held out for full FDA licensure — instead of emergency use authorization — of the vaccines, because according to Chen, it would have delayed their approval and use for several months. “We would be here, at this present moment, without any vaccines being deployed, and having more virus circulate,” he says. “So that could be, I would argue, a worse situation.” 

There are two primary reasons for the pause

The pause is the result of the robust surveillance and safety mechanisms in place, says Dr. Shauna Gunaratne, an infectious disease expert at Columbia University Vagelos College of Physicians and Surgeons. “It doesn’t mean pause as in, ‘stop everything, this is not proceeding further,’ ” she tells Rolling Stone. “It’s just a pause to gather more information to be able to accurately assess whether there is a relationship [between the vaccine and these blood clots] or not, so then we can proceed as safely as possible.”

Chen says that the other aim of the pause is to discuss these rare side effects, educating both the public and those in the medical profession. This way, potential vaccine recipients can be more fully informed of the risks of all Covid vaccines, and know what potentially serious effects could look like (i.e. anaphylaxis, severe headache, abdominal pain, leg pain, and/or shortness of breath). Additionally, medical providers will know how to spot, treat, and report potential adverse reactions to the vaccine — including these rare blood clots. “We’re going to get better information as a result of this,” Chen adds. 

It’s too early to know why all six cases occurred in women

One detail in the FDA and CDC’s statement that stood out was that all six instances of the rare blood clot side effect occurred in women. For now, it’s a bit too early to say why, but according to Eisenberger, it’s in line with what we know about this type of blood clot, known as cerebral venous sinus thrombosis (CVST).

“The cerebral venous sinuses are large veins on the inside of the skull that drain blood from the brain, and getting clots in those is a pretty rare event,” Eisenberger tells Rolling Stone. “I only see it a few times a year — and I see hundreds of blood clots every year. And without a doubt, it’s high levels of estrogen that cause it more than anything else.”  

In most cases, he says, these elevated amounts of estrogen are seen in people who have recently given birth, or who are on combined hormonal contraception (like the birth control pill), and can cause changes to the clotting proteins in the blood, predisposing them to getting blood clots. “And so we’re seeing a synergy between the high levels of estrogen associated with these clotting protein changes, and the ‘pro-clotting’ changes that are occurring in response to the vaccine,” Eisenberger explains. 

But given that there’s still so much we don’t yet know about the six cases of CVST among women who received the Johnson & Johnson vaccine — and that the side effect is so rare — Eisenberger says that this doesn’t mean people should stop taking their birth control, or avoid getting a Covid-19 vaccine. 

Nor does it mean that the vaccine is definitely responsible for causing CVST in all six women, who may have developed the condition with or without being vaccinated. “We paused because of six cases and we don’t know if there are others out there. We probably don’t have the full picture,” Chen says. “I don’t know if there is a real difference in sex — it could be a red herring — but we just have a small sample size, and we don’t want to make too many conclusions, because we don’t have enough information.”

Logistically, this shouldn’t cause a major delay in the vaccine rollout

In an April 13th statement, Jeff Zients, the White House Covid-19 response coordinator on the Johnson & Johnson vaccine, said that the pause “will not have a significant impact on [the Biden-Harris administration’s] vaccination plan,” confirming that to date, the vaccine accounted for less than five percent “of the recorded shots in arms in the United States.” Later that day, President Joe Biden emphasized that despite this setback, having enough of the Pfizer and Moderna vaccines “is basically 100 percent unquestionable, for every single solitary American.”

While it’s too early to tell how the pause might impact the vaccine rollout timeline, given the reported availability of the two-dose vaccines, it shouldn’t set us back that much. “I’m not concerned about it dramatically decreasing our vaccination program,” Gunaratne says. “And I’m not worried that this will prevent us from getting to herd immunity.” 

Johnson & Johnson did not produce an unsafe vaccine

Despite its snappy “one-and-done” descriptor and ability to survive in standard refrigerators, the Johnson & Johnson Covid-19 vaccine entered the scene already at a disadvantage when it came to public perception. It started when the vaccine’s clinical trials were temporarily put on hold in October 2020 to address a possible safety concern, but resumed later that month after no evidence was found indicating that the vaccine caused the side effect.

Then came the findings of the final phase of clinical trials, which indicated that Johnson & Johnson’s Covid vaccine had a lower efficacy rate (66 percent) at preventing symptomatic Covid cases than its counterparts from Pfizer-BioNTech (95 percent) and Moderna (94.1 percent). For a variety of reasons, this figure doesn’t provide a complete picture — especially considering the Johnson & Johnson vaccine’s 100-percent efficacy rate against Covid hospitalizations and deaths. Next was a mixup at a Johnson & Johnson manufacturing plant in Baltimore that resulted in up to 15 million doses of the vaccine being thrown out. 

Which brings us to the current pause in distribution, and concern over whether it might further erode public trust in the vaccine, prompting people to either hold out for the Pfizer or Moderna vaccine, or skip vaccination altogether — leaving our long-anticipated herd immunity still out of reach. And because the Johnson & Johnson vaccine requires one dose and no special deep-freezers, there was hope that it could play a key role in getting us to the herd immunity finish line faster.

But instead of honing in on what people perceive to be downsides of the Johnson & Johnson vaccine, Chen says that it’s important to focus on the fact that we currently have three ultra-performing vaccines. “This is like watching the Olympics and saying that the bronze medalist sucks in their sport. They’re in third place, but that doesn’t mean that they’re not incredible athletes. The gold, silver, and bronze medalists — all of them are winners,” Chen explains. 

The pause’s impact on vaccine hesitant people could go a few different ways

As the initial excitement over the Covid-19 vaccine rollout begins to wind down and we get closer to vaccinating every willing adult in the country, the realization that our goal of herd immunity is in the hands of those who are hesitant about — or flat-out refuse to get — the vaccine is sinking in. According to a recent NPR/Marist poll, one in four Americans said they would outright refuse to be vaccinated against Covid if given the opportunity. 

This is not the same as being “hesitant” about the vaccine: the same poll found that at this point, only five percent of respondents said they were “undecided” about Covid inoculation. So while it makes sense to be concerned about whether the Johnson & Johnson pause might be what puts some vaccine hesitant individuals firmly into the “no way, no how” category, the numbers suggest that in large part, they’re already there. 

But will what Chen refers to as the “big blitz of public messaging” surrounding the potential safety concerns of the Johnson & Johnson vaccine make it a tougher sell to those five percent of people who are still undecided? “I think that a lot of people are wondering whether we’re going overboard,” he says. “The bottom line is that we as the public health community [and] the scientific community, want to be transparent about this. And through this process, we bring up all of the ugly wrinkles — everything about all the vaccines — which some people might end up misinterpreting.”

The alternative approach — which Chen does not endorse — is to ignore or minimize any possible safety concerns as they happen. “I think that that has the negative consequences of sowing seeds of dishonesty, and whether or not authorities are being truthful and hiding information,” he explains. “And so we don’t want to be caught in that particular position.”

Along the same lines, Gunaratne sees the pause as an opportunity for vaccine hesitant individuals — particularly those worried that vaccinations began too quickly and without sufficient research — to see the built-in safety checks of the surveillance phase of the rollout at work. “We’re trying to increase the trust in those who may be hesitant,” she explains. “This is exactly what we expect when we roll out a broad vaccination — the likes of which we haven’t done in decades — and we want to make sure to address everyone’s concerns appropriately, and that even a very low rate [of a potential side effect], like we’re seeing here, is taken seriously.”

It’s also important to keep in mind that all of this — the vaccine development, clinical trials, rollout, and safety monitoring — happened during a global pandemic, the first nine months of which included the challenge of working under a presidential administration that, at times, appeared to be deliberately making things worse. 

“It would be better, obviously, if none of this happened and it was all smooth sailing, but that’s not a pandemic,” Chen says. “The story of a pandemic is that you’re always going to have these opposing forces.”