On Tuesday, the CDC and FDA announced they were recommending that administration of Johnson & Johnson’s Covid-19 vaccine should be paused as a possible connection to rare blood clots is investigated. Multiple states have already announced they’re suspending Johnson & Johnson shots. Of six known rare blood clot cases, one person has died and another is in critical condition. More than 6 million Americans have already received Johnson & Johnson’s vaccine, and regulators stressed the rarity of the events.

This decision comes at a crucial time for the pandemic, as variants of the disease are causing increased numbers of cases and hospitalizations in states like Michigan, Minnesota and Connecticut. Johnson & Johnson’s vaccine has been shown to be highly efficacious, preventing severe Covid-19 cases, hospitalizations and deaths nearly 100% of the time, and the incidence of blood clots now are fairly low. But the decision by U.S. regulators is made easier by the fact that the country has ample supply of both Pfizer and Moderna vaccines. 

“In the United States, there’s a plethora of vaccines at the country’s disposal,” says Isaac Bogoch, an infectious disease specialist and associate professor of medicine at the University of Toronto. “So you don’t need Johnson & Johnson to vaccinate the entire population.”

On a Tuesday morning press conference, the FDA reiterated that there have been no reports of blood clots among the 180 million doses of the Pfizer and Moderna vaccines administered in the U.S. Those two vaccines are based on a different technology than Johnson & Johnson and AstraZeneca’s, both of which rely on modified forms of a common cold virus to deliver the vaccines to patients’ cells. Though not authorized for use in the U.S., AstraZeneca’s vaccine has seen its administrations suspended or restricted in many countries, particularly in Europe, over a similar blood clotting issue. 

It’s unclear how long the Johnson & Johnson pause will last, though at a minimum it “will be a matter of days,” acting FDA Commissioner Janet Woodcock said at the press conference. On Wednesday, an independent vaccination advisory committee will meet to discuss the risks and benefits of the vaccine for the purpose of making recommendations for future distribution. 

Johnson & Johnson has seen its vaccine face a number of obstacles over the past several weeks, after its contractor Emergent Biosolutions had to dispose of 15 million doses that were manufactured improperly. The company has now taken a more direct role in the manufacturing of the vaccine, but it’s expected to take several weeks to get back on track. However, Bogoch doesn’t think these events impacted federal regulators’ decision-making. “The FDA doesn’t mess around,” he says. “They’re a ‘show us the data’ kind of organization. And you know, there are rare, but potentially severe side effects associated with this vaccine, and I think they’re taking this measure based on that.” 

The FDA and CDC say the probable cause of these rare blood clots may have to do with the individual person’s immune response that involves platelets, small blood cells that help with clotting when a blood vessel is damaged. The six cases so far have involved blood clots in patients who have low-platelet counts, which are people doctors would normally expect to have difficulty forming clots. 

For people who received the Johnson & Johnson vaccine more than a month ago, the risk is “very low,” according to federal officials. People who have been vaccinated in the past few weeks should look out for some key symptoms, including severe headache, abdominal pain, leg pain, or shortness of breath, they said, but stressed the reaction was “extremely rare.”

Unlike decisions made in Europe over the past few weeks to suspend AstraZeneca’s vaccine over a similar clotting issue, the U.S. faces less risk of a slowed vaccine rollout due to this administration pause. It “will not have a significant impact on our vaccination plan,” Jeff Zients, the Biden Administration’s Covid-19 response coordinator said in a statement. “Johnson & Johnson vaccine makes up less than 5 percent of the recorded shots in arms in the United States to date. Based on actions taken by the President earlier this year, the United States has secured enough Pfizer and Moderna doses for 300 million Americans.”

“If you’re another country where this is the only option, the benefits significantly outweigh the risk.”

“It’s a setback but it’s not an end of the world scenario,” William Moss, a vaccine expert at the Johns Hopkins Bloomberg School of Public Health says of the pause. “It’s reasonable given the state of the pandemic and the role the vaccine was playing in the United States.”

The mRNA vaccines manufactured by Pfizer and Moderna haven’t been without their own serious side effects: there have been multiple reports of severe allergic reactions to these vaccines, and earlier this month the federal government initiated a study into these allergic reactions. So why pause Johnson & Johnson but not the other two? Bogoch says the situations aren’t that comparable. 

“With those allergic reactions, they almost all occur within the first 20 minutes,” he says. “You can abate negative outcomes with a rapid administration of first aid, and all the vaccine centers are equipped to respond to them immediately. That’s why you have to sit around for a few minutes after you get your dose. It’s not the same.”

By contrast, part of the reason for pausing distribution of the Johnson & Johnson vaccine, FDA officials said, is that doctors and other healthcare professionals need to be alerted to this unusual phenomenon. This is important because if they administer the normal treatment for blood clots, a blood thinner known as heparin, it could cause severe harm or even death. “The pause does allow time for this information to get it,” says Moss. “That is to me a very justifiable rationale.”

That said, health officials and vaccine manufacturers face a unique challenge when it comes to rare side effects like those seen here. “I think if one did the simple math one would deduce that one would save many more lives by vaccinating than you would lose through adverse events,” Moss adds. “So the calculus, I think, is pretty straightforward. The public health messaging is the tricky part.”

Both Bogoch and Moss suspect that as the clotting issue is investigated, more cases of blood clotting will be uncovered. “That will lead to some difficult decisions as to how the Johnson & Johnson vaccine will be used in the U.S. going forward,” Moss says. But the global response by health regulators may end up varying widely, with some countries pausing distribution while others restrict it based on what populations are at risk, while still others simply authorize administration without restriction.

“I can see different countries looking at the same data and coming up with different conclusions,” says Bogoch. “Which is what we’ve seen with AstraZeneca.” A major factor behind how different governments will approach this issue, he continues, is whether it has access to other vaccines. “If you’re another country where this is the only option, the benefits significantly outweigh the risk.”