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This article was originally published on April 13. We are republishing it following the decision to end the pause in the use of the Johnson & Johnson vaccine, including new data on adverse effects.

It’s easy to imagine the internal debate at the Centers for Disease Control and Prevention upon learning that several cases of a rare, dangerous blood clot had been found in women who received the Johnson & Johnson vaccine.

Allowing the vaccine to be distributed while experts reviewed the cases risked exposing more people to the possible problem. Pausing distribution, though, ran a different risk: potentially reducing confidence in vaccines broadly, even the Pfizer and Moderna versions, which use different techniques for building immunity.

It was a difficult choice, but the government opted for the second option. The CDC and the Food and Drug Administration recommended that distribution of the vaccine be paused while its effects are studied and to “ensure that the health care provider community is aware of the potential for these adverse events and can plan for proper recognition and management due to the unique treatment required.”

The government was obviously aiming for as brief a pause as possible, given the ambitious time frame the Biden administration has outlined for vaccinating American adults and the need to quickly build immunity in the population. With a resumption in distribution, though, it’s important to remember just how rare these incidents are.

Since you opened this page, the interactive below has been adding virtual “vaccinations” at a rate of 100 per second. If you see a little red dot, that’s an adverse incident of the type that spurred the pause. (The blue circles indicate something else, which we’ll get to in a minute.) Given that about 8 million doses of the Johnson & Johnson vaccine have been administered and that there have been only 15 such incidents, the rate at which those red dots occur is about 1 in 536,000 vaccinations. It’s not clear what percentage of those who’ve received the vaccine fall in the category that appears to be affected, women younger than 50. But, considering overall doses, a red dot will show up below every 90 minutes on average.

How common are side effect cases?

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That’s another way of saying that the frequency of these incidents is remarkably low. By way of comparison, every year about 12 in 100,000 Americans die in a car crash.

Since the pandemic emerged, more than 570,000 people in the United States have died of covid-19, the disease caused by the virus. That is about 1.8 percent of the 32 million people confirmed to have contracted it. Given the effectiveness of the Johnson & Johnson vaccine in preventing serious illness and death, vaccinating 8 million people could have prevented 142,000 deaths. That number is certainly too high, given that many of those deaths preceded a robust understanding of how to treat the disease and given that many of the most at-risk people were vaccinated with the Pfizer and Moderna shots, which were approved earlier. But it is still the case that protecting millions of people probably prevented thousands of deaths — with 15 problematic incidents.

U.S. health officials called for a pause in the use of the Johnson & Johnson coronavirus vaccine on April 13 after reported cases of blood clots. (Reuters)

Nearly everything we do is a balance between risk and reward. Driving down the street, as mentioned above. Walking outside, where a meteorite could suddenly slam into your skull. Sitting on your couch, where your floor could give way or an electrical fire could break out or a bear could crash through your window. None of these things is likely, so we don’t worry about them, but they could. We draw a balance.

That’s the debate that occurred at the CDC: What’s the balance of the risk of the clots with the reward of vaccinations? Perhaps more important, how does the presentation of that balance affect how people perceive the risks and rewards?

The interactive above shows just how rare the identified problems have been. You probably noticed that it also includes blue circles. If we assume that 1 in 10 of those who were vaccinated would have been infected with the virus if they hadn’t been protected (about the national rate at this point) and that 1 in 100 of them then died of covid-19, those blue circles are the lives that the vaccine saved.