Why the Vaccine Safety Numbers Are Still Fuzzy

Putting a risk in context: The rate of blood clots is extremely low, but the pause in the Johnson & Johnson vaccine could reveal more cases.


By Margot Sanger-Katz and Alicia Parlapiano

When federal officials paused administration of the Johnson & Johnson vaccine after six cases of a rare clotting disorder, one fatal, among the 6.9 million people who had received the vaccine, many critics noted that the chance of a serious ailment was so rare as to be negligible — less frequent than being struck by lightning.

But that roughly one-in-a-million rate is far from certain. Doctors may ultimately find the vaccine is not responsible for the ailment. However, if the two are linked, it’s also possible that the chance of an adverse effect will be higher, even if it remains low.

“Numbers seem quite solid, like, ‘Oh, it’s 10,’” said Caitlin Rivers, an assistant professor at Johns Hopkins University, who studies infectious disease. She said epidemiologists deal with similar matters of uncertainty at the beginning of disease outbreaks. “But they’re estimates, and they will need to be refined, and they may need to be refined a lot, especially since they are small numbers.”

How do we know how common this event is?

If there is a connection between the vaccine and this rare syndrome, new cases are likely to emerge now that the word is out. Regulators announced the pause in part to alert doctors to the existence of this syndrome; as people begin looking, they may be more likely to find and report it. With numbers so low, the addition of even a few more cases could increase the rate. (In the last few days, Johnson & Johnson has reported two more possible cases, one in a woman, and one in a man.)

Cases of rare blood clots

Reported cases of cerebral venous sinus thrombosis in those receiving the Johnson & Johnson vaccine, compared with the number of cases that would normally be expected in that many people over 20.5 days.

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